RPM Threshold [Archive]

Larry Gibson

11-25-2007, 07:36 PM

The RPM threshold is area of velocity where the RPM created by the twist accentuates the defects in cast bullets creating excessive yaw, wobble and pitch of the bullet during flight. Defects in cast bullets are caused by imbalance due to out of round casting, other casting defects, or unwanted obturation during acceleration. Exactly where that threshold is depends on numerous things but those that mostly affect the threshold are; burning rate of powder, quality of cast bullet, design of cast bullet, alloy, fit of bullet to throat/leade and twist of the rifling. I've found over the years that with normal commercial moulds generally available (what most of us use), casting quality bullets of WWs or harder, with proper lube and fit to the throat when using medium to slow burning powders the threshold is in the 125-140,000 RPM range. If we are using the old '06 with a 10" twist and 311291 then the threshold translates to somewhere between 1735 fps and 1944 fps we will find the best accuracy when using 4895 or a slower burning powder.

With a faster powder such as 2400 best accuracy will come at a little slower velocity as the faster powder has a faster acceleration. This means unwanted obturation will occur at lower velocity so accuracy will deteriorate quicker than when the medium/slower powders are used.

The cast bullet bench rest boys really slow twists down which does help considerable. If one is building a special purpose cast bullet rifle then twist is of major importance. My 14" twist Palma barrel (27.5" long) in .308 Winchester shoots regular cast bullets (30-15--FN, 311291, 311041, 311299) with excellent accuracy in the 2400+ fps which is 125,000 RPM, right at the beginning of the threshold. Really, really best accuracy comes down in the 21-2200 fps range which is around the 110,000 RPM area Junior1942 talks about. Same bullets in my M70 target with 12" twist 26" barrel loose accuracy at the same RPM as the 14" twist barrel. My M788 with a 10" twist 22" barrel does exactly the same. With the 12" twist that is in the 21-2200 fps range and with the 10" twist it is in the 1740-1940 fps range.

I've found that most regular heavy for caliber cast bullets shot out of regular rifles shoot best in the 125-140,000 RPM range with medium to slow powders. One merely must know the twist of the rifle to figure what the velocity range for best accuracy will be.

Buckshot

11-25-2007, 08:47 PM

............As an aside but also tied to RPM's is people's (especially the media and polititions) is the belief that the bullet in flight is like a buzzsaw. The big whoorah orver Winchester's original "Black Talon" (ooooh, nasty BAD bullets) with their engineered expansion characteristics had them compared to that buzzsaw turned loose inside someone. I believe it was also this concept that had Janet Reno exclain that the industry needed to develop less lethal bullets. What a bonehead.

The fact is that a bullet or boolit in flight spins at the same rotational speed in forward progression as that imparted by the twist of the barrel. A 10" twist imparts one complete rotation per each 10" of forward motion. An object 16" thick would see 1/2 a rotation of the bullet fired from a 444 Marlin with it's 38" twist.

The image of a slug spinning at 110,000 rpm and expanding as it passes through an object certainly would mess things up. However that 'thing' would have to be 22.7 miles thick to see all 110,000 of those revolutions.

.................Buckshot

wills

11-25-2007, 09:08 PM

Lot of drop in those 22.7 miles.

Bret4207

11-26-2007, 03:30 AM

joeb33050

11-26-2007, 04:09 AM

I think in light of some of the work done by members here we might better call it the "RPM threshold THEORY". It's no more fact than man made global warming or the Easter Bunny from what I see reported. There are of course limits to everything.

What do you mean about the Easter Bunny??? What are you suggesting??? Who else could leave those eggs? Next you'll say something about Santa!
joe b.

Junior1942

11-26-2007, 07:20 AM

I think in light of some of the work done by members here we might better call it the "RPM threshold THEORY". It's no more fact than man made global warming or the Easter Bunny from what I see reported. There are of course limits to everything.I'll go with "theory." I started thinking about rpms when I wondered why my Turk Mauser would shoot jacketed bullets 1 MOA or less at 2400 fps, and the Lyman #323471 wouldn't go 1 MOA at over 1400 fps. I wanted to get 1800 fps minimun for deer hunting with it. But, no, at 1700 - 1800 fps it wanted to put 2 almost touching and 1 out at the edge of the target.

Then I got to wondering why the hole out at the edge of the target usually had a comma on it. Then I got to computing my Turk's rpms and comparing those data with data from the "Fouling Shot's" equipment list. Then, obviously, my commas were the bullets coming apart.

I also agree with Larry in that each rifle and alloy are subject to their own peculiarities. My 1 - 9.5 twist SKS, for example, shoots the 311041 at 1700 fps, 128842 rpms, just fine and dandy.

45 2.1

11-26-2007, 07:27 AM

Shiloh

11-26-2007, 08:07 AM

Is projectile weight part of the equation?? What about boolit length?? The RPM wouldn't change no matter what, but is a short boolit affected more than a long one or vice versa??

In other words, does the RPM threshold theory/affect kick in at different velocities for different boolits?

Thanks folks,

Shiloh :castmine:

felix

11-26-2007, 08:54 AM

Shiloh, yes indeed! Not weight per se, but projectile toughness (uniformity in springback). Things get a lot more OUT of sync as the boolit diameter increases, not as much as the length increases, and more IN sync as the boolit is hollowed out. There is no such thing as a perfectly cast-ed boolit, perfectly sized boolit as per Bob's and Larry's statements. We are just plain lucky that cast boolits can be made to shoot as good as they do per gun style/sport application. ... felix

Larry Gibson

11-26-2007, 10:29 AM

45 2.1

"Hmm, I would think you would have added: imballanced boolits that were shoved out of line by being undersize, too hard or soft for their size and load intensity,"

That is "unwanted obturation".

"wrong powder causing improper balistics,"

Thought that was covered under the explanation of fast vs medium/slow powders effect on obturaration during accelleration. I'm sure a lengthy treatise could be written on the subject or "theory".

"boolits that were not fitted to the controlling area in the case neck, throat and rifling origin among several other things"

Thought I mentioned that with; "fit of bullet to throat/leade and twist of the rifling".

"The main thought is that at a certain RPM, the defects mentioned above allow the centrifigal force to overwhelm the stability of the boolit. The above symtoms are things that the loader introduces into the cartridge and these items can be overcome. The problem here is one of a symtom being thought of as a theory"

Well I don't believe they are "symptoms". The RPM threshold ("theory" if you will) holds pretty much true with the commercillay available moulds (Lyman, Lee, RCBS, Saeco) of normal design that most of us cast bullets from. I have repeatidly stated that. The fact that Bass can get his cast bullet from a custom made mould to shoot well at 2500+ fps out of a 10" twist '06 does not do anyone else any good who wants to get a 311291 to shoot well above 2000 fps in there 10" twist '06. I have shot Bass's bullets with his loading techniques and I do get decent accuracy at 2500 fps BUT the most accurate loads with that bullet are down within the RPM threshold. That is fact, not theory. Same with Starmetal and his 6.5 AR. No one else gets up into the 2400+ fps range with any accuracy what so ever including you. I also have used your loading technique for the 6.5 Swede (we discussed it by PM and on the phone if you remember) and our results were pretty much the same.

As to the RPM threshold being theory; If it is theory then why do the bench resters use custom made barrels with slow twists to cut down on RPM? Why don't they just use the faster twists that HP shooters use? Those faster twist barrels are pretty darn accurate with J bullets, aren't they? Why is it that the best accuracy with ANY rifle using regular cast bullets is going to be below or in the RPM threshold and not above it? Yes there is lots of well proven reasons why the RPM threshold is fact not theory.

How many of you have ANY cast bullet load that gives the BEST accuracy that rifle is capable of with that bullet that is cranking over 140,000 RPM?

Larry Gibson

Bret4207

11-26-2007, 11:00 AM

Larry, I mean no offense, it's just the more things we seem to take as fact with cast seem to have exceptions these days. Why I don't know, it just seems that the more we strive for repeatable "scientific" answers to cast issues, the more it seems this is an art and that some guys manage to break the "rules".

floodgate

11-26-2007, 11:28 AM

joeb...:

And I WISH they'd knock off that "Global" warming junk; all it is is flack promoting the "Round Earth" myth!

floosfate

Larry Gibson

11-26-2007, 11:28 AM

Bret

I take no offense. Yes, there are sometimes exceptions to the rules but when we really look at them we find the exception really isn't there. A good example is the discusion going on regarding the .22 Hornet at 2800 fps. Sounds impressive doesn't it? We might even think of that as an exception to the RPM threshold might we? However when we look closely we find the Hornet probably has a 16" twist and at 2800 fps the RPM is 126,031, just at the begining of the threshold. And if we read the post closely we find that best accuracy comes at a much lower velocity and thus lower RPM also. So, in reality, it is not an exception.

They are also talking the speed one can get with the .222 and that is true, been there done that. However, again if we look close the .222 will no doubt have a 14" twist and 2800 fps is 144,000 RPM. A good Lovern design bullet as they are talking will creep up to that and maybe beyond with reasonable accuracy. You can go much higher in velocity but not with good accuracy. Again, best accuracy with the .222 will be within the RPM threshold.

I believe the scientific "repeatability" of the RPM threshold is self evident. As I asked; how many of us have a regular cast bullet load that gives it's best accuracy above the RPM threshold?

Larry Gibson

45 2.1

11-26-2007, 01:08 PM

I believe the scientific "repeatability" of the RPM threshold is self evident. As I asked; how many of us have a regular cast bullet load that gives it's best accuracy above the RPM threshold? Larry Gibson

Larry, you can believe whatever you want. There have been quite a few people publish articles shooting boolits in 10" or 12" twists well over 2600 fps with under MOA accuracy in commercial rifles and standard boolit molds. Not really hard to do either. Several of the CBA crowd have done it also at one time or another, but thats not what they're about. I suggest you write this up for publication in Joe's new book. Please make sure your name is attached to it. I want you to have all the credit for this.

Bass Ackward

11-26-2007, 01:17 PM

When I was going for Dan's 200,000 RPM contest, I got three successive groups of 1 1/4" to 1 3/4" groups. That was with a Lyman 311440HP mold at 2790 fps. That's 200,000 RPM. But I was not down to Dan's 1" requirement, so I lost.

But I have shot jacketed groups at that velocity that on average were not much better either. That is a factory mold in a factory gun and the holes were round with no do hickeys, tails, and none of the bullets bent or boomeranged back to me either. Does that mean it won't happen to some people? No. Just find out why.

Again: If one guy can do it, then there is a logical reason for any limitation. You have two choices. Ask why, and seek the answer. Or be happy with life and stare at the RPM chart.

Neither choice is wrong. :grin:

leftiye

11-26-2007, 01:39 PM

I have no dog in this fight -as they say. The dog I do have is a PITA anyway. I don't exactly know how to say this, so bear with me. I believe there is something to Larry's theory about RPMs. Even those who might be called "detractors" stipulate to the probability (you'd be foolish not to) of flaws and damage to bullets becoming more critical as rpm increases. Let's not forget that EVERYTHING is becoming more critical as velocity increases and not blame it all on high rpms. The environment in all of its aspects becomes more unfriendly for a soft lead boolit as speed increases.

Larry Gibson

11-26-2007, 08:25 PM

45 2.1

"There have been quite a few people publish articles shooting boolits in 10" or 12" twists well over 2600 fps with under MOA accuracy in commercial rifles and standard boolit molds. Not really hard to do either."

I'm more than happy to continue my learning. Could you reference the publications and dates where I can read said articles? As to not "not really hard to do" then I wonder why so many experienced cast bullet shooters can't do it.

"I suggest you write this up for publication in Joe's new book. Please make sure your name is attached to it. I want you to have all the credit for this."

I'd probably do that except I'm not the one who thought it up.

Larry Gibson

Larry Gibson

11-26-2007, 08:48 PM

Bass

As I recall Dan's test was for 5 consecutive 5 shot goups of MOA or less at 200,000 RPM. No one claimed his prize of a free mould. I've also shot nice "back to back groups" at high velocity but then it goes bad. What happened to the other two groups? I'll also ask how well that bullet shoots in that rifle at lower velocity within or under the RPM threshold? My reason for the question is I continually say the RPM threshold applies to the best accuracy and cast bullet is capable of in a rifle, not acceptable accuracy at higher velocity. Hell, today I pushed the GB C312-150-FN cast of ACWWs to 2600 fps in my Palma. That's 133,790 RPM BTW. I got a few good groups under 2" along the way but the last one at 2600 fps was a 4" group. All the holes were nice and round and it "held paper" but is that acceptable? Of course its not because that rifle will shoot close to MOA with other bullets at that velocity. I'm casting some of harder alloy as soon as I'm done here. I'm looking for a 1 to 1 1/2 MOA load with that bullet in the 2400+ fps range for shooting rock chucks out to 300 yards.

Larry Gibson

andrew375

11-27-2007, 02:04 AM

Well, I'm geting consistent sub moa accuracy with bullets doing 190,000+ RPM. This is with the NEI .224-72gr. in my 1 in 9" Savage.:-D

Bret4207

11-27-2007, 03:26 AM

I believe that at some point a boolit will more or less "vaporize" if subjected to enough RPM. Finding that point isn't something I'm interested in other than in the theory end of things. That being said, I find the experimentation fasinating.

45 2.1

11-27-2007, 04:43 AM

I'm more than happy to continue my learning. Could you reference the publications and dates where I can read said articles? As to not "not really hard to do" then I wonder why so many experienced cast bullet shooters can't do it.

Put out by Wolfe, the publishers of "Rifle and Handloader" back in the 80's or so, out of print. Don't remember its name, but it had a gray cover and was a compilation of articles from the magazines.

Most any high intensity 22 varmint cartridge will do this. I have seen quite a few post about this here. You might contact them to find out how.

As far as why so many experienced cast boolit shooters can't do it, read my tag line.

Bass Ackward

11-27-2007, 05:35 AM

Bass

As I recall Dan's test was for 5 consecutive 5 shot groups of MOA or less at 200,000 RPM. What happened to the other two groups? I'll also ask how well that bullet shoots in that rifle at lower velocity within or under the RPM threshold? Larry Gibson

Larry and All,

Take a deep breath and don't look at this as an attack. It's really for everyone, but I am going to answer your questions too, so it is addressed to you.

As you said, Dan wanted 5, 5 shot groups. The shells were already loaded. I would have had to mold and load more to finish the last 2 groups and the first 5 of that box were squibs for barrel preppers. (clean bore) So three groups was enough to tell me I couldn't do it. And that was the best accuracy I had obtained with that 311440 bullet in this 06. It was simply another light bullet for me to try.

Think pressure. If I wanted to continue to shoot it my style, I would needed a larger capacity case than an 06 and a longer barrel. (extend the pressure curve to raise velocity) (shorter throat too maybe) If I wanted to shoot a heavier bullet at low pressure, I would need more case capacity. So over bore, magnum cartridges make BETTER HV cast guns than standard calibers with this low pressure technique. So .... what makes a good low velocity cast cartridge is different from a HV requirement.

BIG POINT! RPM ceilings will be different based on case capacity and barrel length. A 308 is a better low velocity cast gun, but a 300 H&H would be a better HV cast gun on the same barrel. And if that 300 H&H has a 26" tube, it will have a higher RPM ceiling than an 18" job.

Main point:

Best accuracy with most firearms (always exceptions) and any bullet material is going to be at low velocity. Low velocity = low pressure. Low pressure = low friction. Low friction = low metal fouling, low barrel heat, low barrel vibration. No need for heavy barrels or bedding issues or a long list of stuff. Less recoil too.

Now, try shooting 5, 10 shot jacketed bullet groups at high and a good low velocity load without cleaning. See if your "best accuracy" with jacketed isn't better at low velocity. RPMs .... or pressure? See if your groups don't look more like cast groups too, fliers and all. :grin:

Look at military rifle research and design. Why do you think military groove diameters are usually so generous? To allow for accuracy with high shot strings between cleanings! To cut pressure and friction. That's why military rifles aren't free floated either. Control that barrel when vibration DOES become a problem. German's had steps to do the same thing and barrel bands. So military organizations around the world came to the same conclusion on harmonics / control. You don't have as much harmonics at low velocity.

Now, think about this very closely. If I took 5 different jacketed bullets of 150 grains for HV use. Some would shoot better than others in my gun. Do you agree? My gun can be a sub MOA with one design and an 1 1/2"er with another. Is that the coppers fault? We don't blame jacketed bullet brands for being more susceptible to RPMs now do we? And who simply buys and tests only one jacketed bullet in his gun if he is serious about accuracy? Does my best one mean it will be your best one?

Why then do we insist that every cast bullet design must be capable of high RPM (velocity) accuracy? And when it isn't, we blame the lead. You already KNOW cast is more .... finicky to fouling than jacketed at any velocity. Therefore, if there is a difference between 5 different jacketed bullets performance at HV, then it stands to reason that there would be a BIGGER difference between 5 lead bullet designs performance. Logical huh? And the performance would be different for me than it would be for you, wouldn't it? Explains your use of my bullets now don't it? All you may need is a different diameter from mine. Coarse, it could be the same at low velocity too, but down there at those levels, we think that's normal. And then how many people try just one cast bullet in a gun for HV before they subscribe to the RPM theory? But your straight sided, sharper shouldered 308 case design will never be better for HV cast than my sloping and tapered 30-06 that burns powder more slowly and uses the column as a filler.

Back to .... the "best accuracy" statement for cast. It's really the best "low velocity" accuracy for guns in general then huh? It has a lot to do with our ability to shoot those guns too huh? Has little to do with bullets of any material unless what we or the gun does to them, causes problems like tails, hooks, or bending. So the RPM theory is really a "rifled barrel" theory, not just a bullet or cast one and bullet fit is critical for all bullet materials. A slower twist rate throws less pressure, has less friction and heat, and therefore less vibration. All more cast friendly elements and why lead performs better. Pressure. Not RPMs. (Catch that Junior)

Take your time and read this through again as it is my last attempt. I have said all there is to say without beating it all to death. And since "pressure" is my theory and can't be proved, it's no more provable than your RPM theory. Believe what you want to believe. But if you work with a pressure ceiling, you will find far more answers than if you go at it from an RPM standpoint.

Bret4207

11-27-2007, 07:09 AM

[QUOTE=45 2.1;250723Put out by Wolfe, the publishers of "Rifle and Handloader" back in the 80's or so, out of print. Don't remember its name, but it had a gray cover and was a compilation of articles from the magazines.

.[/QUOTE]

The Art of Bullet Casting?

Bass Ackward

11-27-2007, 08:01 AM

Well, I'm geting consistent sub moa accuracy with bullets doing 190,000+ RPM. This is with the NEI .224-72gr. in my 1 in 9" Savage.:-D

Thanks, Andy. The more people speak up the better the statistics for those that have the BS flag in hand.

What powder? 4831?

felix

11-27-2007, 08:18 AM

The problem here is trying to determine which variables are independent of one another for maximizing/minimizing something. From this conversation it is apparent that we are trying to maximize RPM while at the same time minimizing group size. Here is where the proverbial cluster analysis of all the variables, independent and dependent, are grouped into one space to display the shape and density of the cloud of data points. Yes, JoeB and Maven, I have to bow to you both in this instance, in that statistics is hereby needed big time. The objective of this study would be to determine which variables are actually independent on one another so that they can be controlled to optimize the path towards realizing the two "goals". On the other hand, when trying to optimize only one of the two goals the problem becomes much more manageable, if not more realistic. ... felix

Larry Gibson

11-27-2007, 10:01 AM

Well, I'm geting consistent sub moa accuracy with bullets doing 190,000+ RPM. This is with the NEI .224-72gr. in my 1 in 9" Savage.:-D

Kindly show us pictures of the groups (if available) and load data so we may codgitate on it. Thank you.

Larry Gibson

Larry Gibson

11-27-2007, 10:36 AM

Bass

The question goes like this; "Why can I get the same accuracy as I get out of my '06 with 311291 at higher velocity, like at 2400 fps, than I get at 1850 fps?"

In other words we are not talking different bullets, different barrel lengths, different cartridges or different twists. What we're talking about is that guy, or any guy, is not going to get the same accuracy at 2400+ fps with a 311291 out of a 10" twist '06 that they get at 1850 fps. Now if the guy is getting 1 1/2" groups at 1850 fps he can shoot so boosting the velocity and being inaccurate does not mean it's the guy who can't shoot. The reason for the inaccuracy is RPM.

You dance around the answer with your dissertaion on lower velocity means lower pressure which you say means better accuracy. That is very seldom true with jacketed bullets as best accuracy comes with efficient shot to shot ignition of the powder. It can be had, with most any rifle, at low velocity, medium velocity or high velocity. Those ranges of velocities depend on the cartridge of course. I believe most here know it.

But to the point; lower velocity mean lower RPM with a given barrel twist. Pretty simple isn't it. Shoot the same cast bullet loads in rifles of 10, 12 and 14" twists and you will see that the slower the twist the better the accuracy. Now please don't tell me that the pressure is different in the different twists. The difference in pressure between those twists falls within shot to shot variation which means it is essentially the same. Even if you vary the pressure with slower powders the accuracy deteriorates within the same RPM threshold in each of the barrels.

RPM effects jacketed bullets the same as it does cast bullets. The inaccuracy is just not as noticeable because most JBs are made quite well and they do not suffer the adverse obturation during acceleration that cast bullets do. Yes jacketed bullets can be pushed to far too but that's another dissertation. If you remember the reason for the slower twists with Palma barrels in 7.62 is that service ammo was required. The slower twists give less RPM to the 145-155 gr ball bullets and the imbalances (defects) are not as accentuated. Inshooting service issue ammo they could not vary the loads. They could vary the twist and lower the RPM. That reduction of RPM greatly improved the accuracy of the service 7.62 ammo. It is the very same thing with cast bullets. If you want to drive them fast then lower the RPM.

Higher RPM accentuates the imbalances of the cast bullets which causes less accuracy. To get the best accuracy at higher velocity you must change the RPM by going to a slower twist. If not then your best accuracy will not come above the RPM threshold.

Larry Gibson

Idaho Sharpshooter

11-27-2007, 10:43 AM

45-2.1,

that is the beauty of this forum, you can support (or not) any theory that comes down the road.
But, when the umpire says "strike three..." Mighty Casey has struck out! The RPM threshold computations and testing protocols were ones done at great taxpayer expense at Picatinny Arsenal several years ago. That data was adopted by Sierra and is shown as their information supporting the fact that bullet MV provides up to three separate ballistic coefficients as the bullet moves and slows down. One of the other things they got was that the decay rate of the spin changes less than one-percent over the course of the bullet's flight.

For those (like me) challenged by Quantum Mechanics and String Theory, the simple answer is: take a soda can and draw a circle that size on a piece of paper. Draw lines up & down and left to right, like crosshairs on a rifle scope. Where the NS and WE lines intersect is the actual geometric center of that bullet. Call it C1. Next make a dot, say .002" away from the center crosshairs. Imagine that dot is a microscopic void in the bullet. Call it C2. 180-degrees away from that void is the center of gravity, call it CG.

Step Two: take a rifle barrel chambered in, say 308W, with a 1:12" twist. That twist barrel will provide that bullets 1RPR (revolution per rotation) every 12" inside the barrel. Shoot that bullet at 3,000fps. At exit that bullet is traveling that speed and rotating at

Let me call my friend at Sierra and get the exact formula phrasing.

Rich

Here we goes; that bullet is traveling 3,000fps and revolving at 1rps. Multiply the speed times the rps and then multiply that times 60 (seconds per minute). 1x3,000= 3,000 x 60 = 180,000rpm. Once the projectile leaves the confines of the barrel C2 and CG come into conflict. Each wants to be the center that the projectile rotates around. That can't happen. The projectile begins to rotate back and forth on it axis trying to center both at the same time. So, about 180,00 times per minute the projectile yaws back and forth the distance between the two centers. Visualize a minute hand on a clock constantly moving between 10am and 10:00:01am...180,000 times a minute. That movement is what causes the bullet to yaw off course in a random pattern, and adds to the droop gravity is causing. The bullet may be structurally sound enough to endure that for the several 1,000ths of a second it takes to get to the 100yd target. Exaggerate the the difference between C1 and C2 and CG enough and the projectile fails.

felix

11-27-2007, 11:10 AM

Larry and John, I think you both are really saying the same thing in a round-about-way. Pressure, with land acquisition especially, destroys boolits more than bullets, making the RPM factor much more paramount. I wish there was a good way to measure the pressure curve, projectile deformation, and muzzle deflection simultaneously at each milli/micro-second level. Something like what the oil finders do daily with their geophones (multiple microphones and explosives), and the medical folks do with their magnetic wave shooters and re-capturing devices. ... felix

felix

11-27-2007, 11:18 AM

IS, the question is not how much the rotational speed slows, but how the boolit acquired that rotational speed and it's effect on the pressure curve and muzzle dynamics. Yes, we must include all of the known measurements for that barrel in question to be able to decipher their effects in detail. ... felix

joeb33050

11-27-2007, 11:56 AM

I went through the CBA 2006 National Match results, made a spreadsheet showing the TWIST, MV and RPM of the entrant's guns.
Taking out those with gain twist, then the SS Breech Seaters, the RPM varied from 158,400 down to 96075, with an average of 125578.
The workbook is attached. Would someone translate so it can be shown here?

There's a lot on the 30BR and short bullets and slow twists at http://www.6mmbr.com/30BR.html

I suspect that short light bullets are easier to shoot accurately, hence the references to 100-125 grain .30 caliber bullets.

Greenhill says that in 30 caliber, a 10" twist will stabilize a bullet 1.42" long, which would weigh ~250 grains. It says that a 15" twist, for examploe, will stabilize a bullet .95" long weighing ~166 grains. All Cast bullets.
I think it's clear that 30 caliber twists are generally "faster than needed", that slower twists with shorter lighter bullets are easier to shoot and torque on the bags less.
My impression is that gain twist and slow twist barrels are "easier on the bullet" as the bullet accelerates in rotation.

joe b.

Junior1942

11-27-2007, 12:10 PM

Joe, take out the ones with estimated velocities. Use only the ones with chronographed velocities.

Bass Ackward

11-27-2007, 01:09 PM

Even if you vary the pressure with slower powders the accuracy deteriorates within the same RPM threshold in each of the barrels.
Larry Gibson

No sir. Not in any gun I ever shot cast in.

The barrel provides the accuracy. Twist rate is a "maximum" bullet weight and range distance related factor for stability. Bedding and barrel dimensions control the harmonics. If you can't build a rifle to control the harmonics, you must slow the twist rate.

The 8.66 twist, 7MMs favorite bullet was the 115 grain Speer with 45.5 to 46.5 grains of 4064 in all of them. Trick was that you had to bed them with enough tip pressure to control the vibration.

Larry Gibson

11-27-2007, 02:02 PM

Bass

The harmonics of the barrel have absolutely nothing to do with the defects in cast bullets as we cast them or the defects (unwanted obturation) caused by accelleration. It is those defects that are accentuated by RPM and cause inaccuracy in the exterior ballistics portion of the bullets flight. I''ve not found harmonics to play that much of a role in accuracy. A good barrel that is free floated will shoot well. Good harmonics of a barrel are directly tied to consistant ignition and burning of the powder. That translates in consitant movement of the bullet down the barrel shot to shot. That means the vibrations (harmonics) of the barrel are also consistant shot to shot. A barrel doesn't have its own harmonics as such. Consistant loads make for consistant harmonics in the barrel. Velocity is imaterial to consistant harmonics.

As to if I can't build a rifle some would consider that a cop out on your part if you infer only custom guns shoot cast bullets fast and accurately. I am saying the RPM threshold holds to any rifle, custom, factory or military.

I've absolutely no idea what the 7mm with the Speer J bullets has to do with this thread.

Larry Gibson

Bass Ackward

11-27-2007, 03:05 PM

Bass

The harmonics of the barrel have absolutely nothing to do with the defects in cast bullets as we cast them or the defects (unwanted obturation) caused by accelleration. Larry Gibson

Larry,

I bought Veral Smiths little Blue book that has cost me several thousands of dollars to date. My first purchase was a 700 Remington LSS in 30-06. First thing I did was to free float it. Next thing I did was try to shoot high velocity cast in it and I stuck the bolt. 300 rounds later I started HV work again.

I used the 56 grains of RL 19 that I always quote as this is where groups peaked. Group sizes were about 2 " with defective fliers running out to 6". So I bedded it with 2# of pressure. Groups came in a little to let's call it 1 3/4" and fliers stayed about 6". I upped the pressure to 4#. This time groups stayed the same and fliers came in @ 4". Hmmmm I says. So I upped the pressure to 8#. Groups came right in to about an inch and fliers now are 1 1/2". That's a bullet with the grand canyon running up the side.

My "guess" is that a bad bullet or load causes a rotational change that alters timing and that change alters barrel vibration causing the bullet to exit at wild points. Uncontrolled, that vibration is going to get worse the higher the velocity goes. Bedding pressure minimizes that effect. The only time I get truly wild fliers now is when my lube thickens and sizes down my bullets enough that they DO let go of the rifling. Then every shot is a flier, good bullets and all. But based upon my results above, I disagree that bad or unbalanced bullets are the cause of most fliers. Otherwise bedding would show no change.

MOST of all cast bullet damage is done from internal ballistics while the bullet is still in the barrel, and not from external ballistics at least until longer range. Even at high RPM.

Idaho Sharpshooter

11-27-2007, 05:56 PM

Actually, everything we do to a cast boolit after it falls from the mould onto a terry cloth pad is bad for it. One reason the breech seater boys shoot so well, is they do nothing to the bullet until they seat it into the barrel with a .0003"-.0005" interference fit that has the effect of sealing the the bullet off from ignition gasses and causing deformation as the bullet obdurates to fill a throat and then pressure swages back down to fit the rifling. Their bullets are pre-obdurated, for lack of a more precise scientific term. Their bullets are also softer, which would allow a smaller void to be swaged closed inside the barrel. J-bullets are swaged twice or three times in the forming process...which pretty well swages any imperfections, occlusions, whatever term you prefer; out. Plus that copper skin is about 100Bhn, and they are always swaging an increase is diameter which helps.

Rich

felix

11-27-2007, 06:35 PM

Rich, what you are saying is fine, but pay attention to these definitions:

1. ob·tu·rate: To close or obstruct.

2. ob·du·rate: Not giving in to persuasion; intractable.
Synonym: inflexible.

... felix

AnthonyB

11-27-2007, 06:40 PM

Guys, this is a fascinating thread. Please don't let the different opinions halt the discussion.
Tony

Rrusse11

11-27-2007, 07:18 PM

Guys, this is a fascinating thread. Please don't let the different opinions halt the discussion.
Tony
I'll second that! :)
My 2 cents; handmade cast boolits can rarely be as consistent as swaged/jacketed, lower rpms and slower powders mean less centrifugal deviation for cast. My slow twist 444 and 454 Colt Marlins have a speed/RPM threshold with heavy boolits that is clear. Never tried to test the upper end.
Cheers,
R*2

Larry Gibson

11-27-2007, 09:21 PM

Bass

Whens the last time you EVER saw a bench rest rifle with fore end pressure on the barrel, even cast bullet bench rest rifles? If what you say is correct then all of them ould have pressure on the barrel instead of none of them. Those boys shoot to win. If it works they use it, if it doesn't they don't. The only time I've seen or heard of pressure on the barrel helping accuracy is with bad barrels or really light weight ones. There are muzzle attachments that tune the harmonics to the load or you can tune the load to the harmonics. However, good barrels shoot well without either when consistant loads are used. Had a fellow HP shooter who had a target rifle (6.5-308) that shot 1/2 MOA with MKs. He thought if he put a Browning Boss on it he could get 1/4 MOA so he had one put on. Well he tuned and tuned and tuned and couldn't get the Boss equiped barrel to shoot under 1 moa. Took the Boss of and it shot 1/2 MOA again. So does this one incident mean the Boss doesn't work? No, it just means that good barrels shoot well and in this case the Boss did not lesson the harmonics but increased them.

If you can keep a bullet with a "grand canyon running up the side" from being a flyer just by applying pressure to the barrel then you must part your hair in the right spot. I'd think you be too busy doing that magic (and making a fortune) to all the bench rest rifles and match rifles out there to be discussing this with me. But let us continue our conversation anyway.

"MOST of all cast bullet damage is done from internal ballistics while the bullet is still in the barrel, and not from external ballistics at least until longer range. Even at high RPM."

Bass, now when have I EVER said RPM damages the bullet? I haven't, and at normal velocity I don't think it will unless there is a gross defect in the bullet from some other reason. Of course the damage to a bullet is done in the barrel, that's called obturation. Obturation can be good or it can be bad but most obturation is bad because it creates inbalances in the bullet. The inbalance means the center of gravity does not coincide with the center of form. While in the barrel the bullet rotates (regardless of the twist) around the center of form. When the bullet exite the barrel the bullet rotates around the center of gravity. If the two do not coincide the bullet is inbalanced. It is during the external ballistics (that's outside the barrel) that RPM accentuates the inbalances creating yaw, pitch and wobble. The centrifical force created by the higher RPM causes the yaw, pitch and wobble to be worse the higher the RPM is. This translates into larger dispersion or larger groups. There is a very good description of this (pictures too) on pages 16-19 in Hornady's Fourth edition, Vol 1 reloading manual. Pressure on the barrel will have no effect on how the centrifical force causes this dispersion during the bullet during it's flight.

I shot your bullets too in a 30-06 rifle with 10" twist that is MOA capable with several loads both jacketed and cast. Using your load of 56 gr RL19 (actually worked up from 54 through 58 gr) and groups ran from 2.4 to 14". Most were in the 3-4" range. I actually got better accuracy with H4831SC of 2-3" groups in the 22-2350 fps range. But guess what? I also shot your bullet over 27 gr of 4895 (that's my accuracy load with the Lee C312-155-R). Your 154 gr LBT bullets ran 1905 fps/137,160 RPM and went into .98" (call it 1"). So what's that prove? Nothing much except your bullet shoots much more accurately down in the RPM threshold. That is just what I have been saying.

So, yes I can get "reasonable" accuracy of 2-4" (if we call that "reasonable" with a MOA capable rifle) at a higher velocity of 2000-2400 fps with your bullet and your load. However, best accuracy of MOA (consistant accuracy also) comes in or under the RPM threshold. The higher the velocity the higher the RPM and the larger the groups. Not to hard to understand.

Larry Gibson

joeb33050

11-28-2007, 03:37 AM

Joe, take out the ones with estimated velocities. Use only the ones with chronographed velocities.

There's a column on the sheet that's either C = chronographed, or E = estimated. I wish somebody would put this up, here.
joe b.

Pat I.

11-28-2007, 03:56 AM

It seems as if this topic could go on forever with no agreement but I think if we look at what jacketed BR shooters are using for bullets it takes the theory out of the equation. The bullets they use are made on J4 jackets which have .0002 run out on the jackets compared to maybe .0005 with factory jackets. The reason they use them is to keep the CG and the center of form as close together as possible to maximize balance. It has nothing to do with internal ballistics and is all about the bullet once it leaves the barrel.

I suppose it might be possible to attain usable accuracy with a cast bullet at speed but I'd be willing to bet that in most if not all instances best accuracy will be found by following the basic principal of an RPM threshold and I also believe it could be predicted for twist rate and caliber with a little study. It has nothing to do with the bullet vaporizing or anything else, it's all about velocity and spin rate accentuating the imbalances built into cast bullets by design.

My 2 cents.

Pat

45 2.1

11-28-2007, 04:54 AM

Rich-
The government does fine work, usually to prove a point and isn't shy about throwing money at things. The point is they didn't do that work with a bunch of cast boolits though. I've seen enough things that those ideas don't explain to know that it's only a theory. Until a theory explains everything and is repeatedly provable, it's only a theory and not a law. Too many articles busting it have been published besides the things i've done and seen for me to accept it covering everything. Too much smaller caliber work going on for a long time that is repeatable to believe RPM theory.

This RPM theory thing seems to be evolving before our eyes as the parameters change with every post.

felix

11-28-2007, 06:14 AM

Everything is RPM limited, depending on its mass construction and its diameter. ... felix

Idaho Sharpshooter

11-28-2007, 09:10 AM

45-2.1,

until I built my 22-378 Wbee Ackley Improved I would have agreed with you. But, since I was able to blow up every bullet I was offered with enough speed; and that case could hold enough powder to do so, I have to stick with what the arsenal and my personal testing has shown me.

Going to the .223 as a primary infantryman's weapon back in the 1960's may make more sense today in light of the RPM issues. Small caliber, and heavy for caliber bullets = fast(er) twist. Faster twist and 3,000+fps MV = lots of RPM. Minute %-age of spin rate decay = increased radial/axial tissue displacement on impact.

Rich

Larry Gibson

11-28-2007, 09:27 AM

I copied Buckshot's response on another thread here because it's a classic example of the effects of the RPM Threshold and gives a good view as how it is misunderstood.

"A shooting buddy has a Winchester M54 in 22 Hornet. He can shoot cast almost as fast and accurately as his jacketed loads. Well, to about 2500 fps anyway . Heck, he even shoots pulled 40 gr 22RF bullets. These over light charges for fun and the occasional crow in his back yard.

I've shot cast 56gr boolits form my Savage M112 single shot .223 accurately to 2200 fps. It has a 12" twist. Actually the accuracy is still useable at 2400 fps but it isn't going to win any matches..................Buckshot"

A lot of casters, experienced and inexperienced, say something like "wow, he is shooting cast bullets accurately a lot fast than I am in my '06 and 6.5 Swede. Wonder why I can't do that." They can tweek the alloy, lube, sizing or case neck thickness all they want but the still won't get there ("there" being accuracy with cast as good as with jacketed at "speed"). The reason why, if we bother to look closely, is that a M52 in .22H has a 16" twist. That 2500 fps load is only cranking 112,528 RPM, close to the bottem of the RPM threshold. The Savage with the 12" twist at 2200 fps is cranking 132,000 RPM, up into the RPM threshold. They go over that and accuracy is lost. The .22H, in my experience since '75 with the Hornet and cast bullets, is that he is maxed out case capacity wise with a slow powder. Switching to a faster powder won't help but maybe an alloy change will alow another 100 fps or so. With the 12" .223 a change to a slower powder, an alloy change or different sizing might also get him another 150 fps before accuracy falls off but that's questionable as he already is well up into the RPM threshold.

Those are good example of how we see someone getting higher velocity with accuracy than we do and we think we should be able to do that also. You can IF you have the same twist. However, if your rifles barrel twist is faster you won't. It's all about RPMs when we want to go fast with accuracy using cast bullets.

Larry Gibson

felix

11-28-2007, 11:11 AM

More ammo for this discussion: The BR games during the 80's in the Houston Warehouse concluded without doubt the 22's are more accurate than the 24's by at least 1/3. Problem is nobody can realize that accuracy within realistic ambients. Folks who could shoot that good never chanced it, and I would not either considering the costs of attending the matches. There is a big difference between 22's and 24's, 50 grains versus 70 grains, where each is twisted at the preferred 14. ... felix

cbrick

11-28-2007, 05:11 PM

Fascinating thread and quite a can of worms. Here's some more food for thought.

Has anyone given thought to it being normally easier to get better accuracy at higher velocities with smaller diameter bullets (say the Hornet and 223 discussed in this thead aside from the well discussed twist rates? The "theory" being that any possible defect in a smaller bullet, be it from casting, sizing, loading or firing (internal ballistics), is closer to the center of axis during free flight (external ballistics) than it could be in a larger diameter bullet. This coupled with the bullet being lighter in weight it also has less mass spinning off center regarless of RPM's. Sectional denisty was the "key" referenced in the previously mentioned Handloader article.

Rick

leftiye

11-28-2007, 06:36 PM

Any given size and shape defect is also proportionately bigger in a smaller boolit. This means that the smaller boolit must be more perfect- have smaller defects- to be as precise, relatively as a larger boolit. Granted it is closer to the axis, and recieves less centrifugal force. Another factor favoring smaller bores is the lower sectional densities of their boolits (barring extreme designs). They will tolerate both faster accelerations (higher pressures) with a given alloy, and more RPMS (as you state).

felix

11-28-2007, 07:00 PM

Must weigh each 22 boolit to get high speed accuracy, dividing them into 1 tenth grain offset piles. The lower weight piles must be remelted. Take only three piles. If those three piles do not amount to 90 percent or thereabouts of all the boolits from that pot run, strongly consider another pot run. Segregate multiple pot runs using the same lead or not. A pot run is defined by each cold start. ... felix

manleyjt

11-28-2007, 08:02 PM

I will add something not mentioned and may not be of any value to this discussion, but here goes anyway.

In Robert Rinker's Understanding Firearm Ballistics there is a discussion about the Greenhill formula and the constant that is used. It is on pages 141-142. They make an adjustment for density of bullet material and also for velocities greater than 1800 fps (stated in the text as a change form 150 to 180 to account for a 20% slower twist and a 20% longer bullet).

anyway I don't know if this adds anything to this discussion or not. I had wanted to post this in another thread that discussed this topic, but did not.

joeb33050

11-29-2007, 05:24 AM

There's a lot of "why it does or doesn't exist" here, but little that I can see about demonstrating that it, the "RPM Threshold", does exist.
CBA records certainly include high RPM cases, although the average is lower.
I'll get some more CBA records put together.
Could the threshold merely be a combination of accepted twist rates and accurate velocities?
If the support is history, "The good shooters don't (often) exceed XXXXXX RPM.", could that be because of 12 twist 30 caliber barrels and the ~2000/2200 fps accurate velocity ceiling?

There's some reason that the SS breech seating guys shoot as well as or better than the fixed bolt guys in CBA competition. I don't know what that reason is, but have always imagined that bullet jumping forward AND having to start spinning, all in an instant.
Something makes it necessary to use harder bullets at higher velocities; could it be the damage at the initial jump on softer bullets?

Maybe the secret of CB accuracy lies in the gain twist barrel, from zero twist at the chamber end to minimum required twist at the muzzle.

Where can I read about the alluded to government testing?

Thanks;
joe b.

Bass Ackward

11-29-2007, 06:13 AM

Bass

Whens the last time you EVER saw a bench rest rifle with fore end pressure on the barrel, even cast bullet bench rest rifles? If what you say is correct then all of them ould have pressure on the barrel instead of none of them. Larry Gibson

Larry,

Time for another deep breath. Bear with me. Every time you have to stop and think about what I wrote, think about why you NEVER see a free floated military barrel.

You said yourself, jacketed accuracy can be found at low, medium or high velocity. Jacketed throws enough friction that as the speed increases, the vibration speed increases and the barrel whip is minimized. No bedding is needed because you simply alter your powder charge or your powder speed to get the vibration pattern needed for consistent shot to shot exits.

The wine glass with your finger routine. Slow and easy and the pitch is low and soft. Push harder and the pitch of the tone gets higher and louder. Ever play the geetar? A real lose string can be seen vibrating and drawn tight never seems to move.

With lead, you must stay with the slowest powder producing your best velocity or accept a lower velocity (RPM). So jacketed accuracy is a different bedding game. Stop here. You have to believe this or your wasting your time reading on.

Lead bedding logic SHOULD be different. It doesn't "have to be" if you can get best accuracy within the harmonics range you want to operate in with the barrel you have. (HV 30 caliber seems to be 2200 to 2400 fps) Bedding won't help then if you are accurate and it could make things worse. But lead, you can only throw so much friction before your lube breaks down and you lead. SO if your best (most accurate) load effort CAN"T reach the proper harmonics, then you MUST change the oscillating frequency which may be more pressure or more pressure farther out. Bedding does this. The farther out vibration is controlled, the less pressure is needed. But if you don't tune your lead, you live with the result.

(It's the same with jacketed and thus the 115 grin Speer reference in the 8.66 twist. While people believe it's the twist rate overstabilizing, it's the increased barrel FREQUENCY from speed (the finger on the wine glass again) making bullet harmonics and bullet exit at a single point (accuracy) even MORE difficult. We prooved in several rifles that light bullets COULD BE shot MORE accurately than heavy for which the 8.66 twist was designed. All we did was change bedding.)

Now, when a lead bullet reaches the point that lube breaks down and it .... grabs, that barrel and the vibration pattern is changed dramatically. (You pushed your finger harder on the wine glass and got a louder sound) So you get a flier. If you bed so that that change is trivialized, then you get less of a flier. Or so that is my logic. Same thing for minimizing humidy effect on lube and temperature, and metal fouling too. This is why militaries grab their barrel and hold it so as fouling alters accuracy, the effect is trivialized. A benchrester simply cleans his gun. Bedding does externally what a powder or charge change does. It's as simple as that. Maybe not as well, but well enough for lead.

Works as long as you aren't destroying bullet because of fouling, then all bets are off. You have to understand once you reached that point. Based upon my 35 Whelen tests last month with 4-6 BHN harder bullets and how that affect accuracy, I am going to get a .308 sizer and harden my bullets a tad and see if I can't shoot HV this winter.

When you slow the twist rate, you are in effect cutting pressure that alters the vibration pattern to allow higher velocity (RPM)s before the lead becomes unstable changing vibration again. Why not bed?

One more point: Larry "So what's that prove? Nothing much except your bullet shoots much more accurately down in the RPM threshold. That is just what I have been saying."

No all that prooves is that your gun shoots that bullet more accurately at that velocity. Free floated aren't ya?

Bass Ackward

11-29-2007, 06:43 AM

To those who feel this is an argument, I am sorry. It certainly is NOT on my part.

And while I am speaking to Larry, I will get to SHOW him later on when he comes out. I am really speaking AT everyone else. Minor subjects here never address a cast bullet gun. Only the bullet and load.

But I believe a cast bullet rifle is a different horse from what makes a good jacketed rifle and I have stated bits and pieces of this over the years, and Larry is the first to call me on it, forcing me to elaborate.

There are cases where we get a rifle that just does well with cast cause the conditions are just right. When they are not, we live with the RPM chart.

Sorry if I ofend anyone with my lecturing which is really just trying to express myself in as little space as possible.

andrew375

11-29-2007, 07:42 AM

Thanks, Andy. The more people speak up the better the statistics for those that have the BS flag in hand.

What powder? 4831?

VV N120, Blue Dot, and N135 so far.

andrew375

11-29-2007, 08:23 AM

Kindly show us pictures of the groups (if available) and load data so we may codgitate on it. Thank you.

Larry Gibson

Well [url=http://castboolits.gunloads.com/showthread.php?t=11228here's one [/url]I posted recently. Alright the bullet is only doing 168,000 rpm, but I've had the same performance at around 2400fps but with "slower" powders, principally Blue Dot and N120. The reason why this target is with N320 was that it is what I was loading at the time. I don't use the faster loads due to the increase in barrel heat and expense. If you choose not to believe any statement I make regarding my experiences then you may do so, though why I would make up the above statement is beyond me.

The reason why I responded to statement about maximum rpm limits is because the reasoning seemed a bit sparse. There could be any number of reasons why accuracy suffers at higher velocities, the main one has been shown to be failure of the bullet material to withstand the pressure. Dave Scovill wrote an excellent piece on matching bullet alloy strength to pressure in HandLoader. I've been shooting cast bullets for nearly thirty years and I have encountered all sorts of folk lore about cast bullets and what you cannot do with them. For example: .22 bullets are virtually impossible to cast, cast bullets are no good in a .223 because I'll be cleaning the lead out after each shot, they wont shoot in a fast twist as they will be ripped apart, etc. All of which I (we) know are false but I get lectured on by "experts" quite regularly.

BTW, I do not do extensive group shooting, just enough to determine that the load is worth bothering with, so I don't have an extensive archive of targets. I am principally a belly shooter: no rest, just a single point sling and glove. Very few of our ranges are fitted out for proper bench rest shooting and whilst I have a proper front rest and set of bags the stock on my savage, a McMillan Anschutz Prone, definitely is not a "bag rider". So unlike if were into bench rest I am not particularly "anal" about group sizes. Maybe I should be, who knows?:twisted:

As I said in the posting referred to above the rifle shoots groups like this just to keep me in my place, any poor quality shooting is purely down to the nut behind the trigger.

felix

11-29-2007, 08:44 AM

Andrew, find some N105 and use as BlueDot exactly by weight. Wider temp range accuracy, cleaner, and the same accuracy on a day well suited to BlueDot. ... felix

felix

11-29-2007, 09:10 AM

Joe, typically, gain twist will work against a boolit with quick acceleration. Would require deeper barrel grooves, keeping everything else the same. Shorter the boolit, the better the boolit will withstand the non-linear increasing rotational rate. Breach seating wins hands down, provided the breach seating technique keeps the boolits entirely concentric throughout the ignition phase. ... felix

Chargar

11-29-2007, 09:12 AM

Well, I certainly don't have any firm knowledge on this area, but that doesn't prevent me from adding my contribution to the fire plug.

Is it pressure or RPM.. that cause things to go South? Well, without pressure there are no RPM. More pressure produces more RPMs. So which is it?

I have a gut feeling that both play a part when juice on the bullet passes the point the alloy can withstand. The pressure pushes on the base, and the RMP rip at the sides of the bullet. They act together to kill the accuracy potential of that once wonderful cast bullet.

That are so many variables that I doubt if a fellow could codify the ceiling for RPMs or Pressure. Different cartridges, barrels, powders, and alloys will roll snake eyes at different points.

Junior1942

11-29-2007, 09:56 AM

I'd like to see some slow motion photos of a cast bullet going 150,000 rpms about 25 yards from the muzzle. I bet we'd see why my target holes had commas.

felix

11-29-2007, 10:07 AM

Charger, that is why multivariate analysis (cluster analysis, ect.) was invented. You can't codify these parameters without the proper equipment, and even if you did have the equipment, the time it would take to get a mess of data for the "cloud" would drive a person without the necessary incentive(s) absolutely nuts. ... felix

Larry Gibson

11-29-2007, 11:22 AM

I have a gut feeling that both play a part when juice on the bullet passes the point the alloy can withstand. The pressure pushes on the base, and the RMP rip at the sides of the bullet. They act together to kill the accuracy potential of that once wonderful cast bullet.

That are so many variables that I doubt if a fellow could codify the ceiling for RPMs or Pressure. Different cartridges, barrels, powders, and alloys will roll snake eyes at different points.

Pressure and RPM do go hand in hand but the effects are different. Take pressure; Load a '06 with 311291 to a pressure where obturation is excessive with and then load to the same pressure with 4898. The pressure is the same but the time pressure curve (acceleration) is slower with 4895. That means the 4895 load is not causing the excessive obturation yet and can be driven to a higher velocity before it does. Thus accuracy deterioration will happen at a lower RPM with 2400 than with 4895. This is why Bass talks of using the slow burning powders and he is correct. With the slow burning powders you can push a given bullet faster and maintain accuracy. So while pressure and RPM are related there is a difference in how one gets to pressure. Get there too fast and RPM will be low. Take your time getting to pressure and RPM can be higher.

Larry Gibson

Larry Gibson

11-29-2007, 12:45 PM

Et all; As Bass said this is not an arguement though it may seem that way to some. We are having a serious discussion here. As Bass said, we are bringing out lots of things we have learned and observed over the many years both of us have been casting bullets and shooting them.

Larry,

Time for another deep breath. Bear with me. Every time you have to stop and think about what I wrote, think about why you NEVER see a free floated military barrel.

Took the deep breath and have been thinking about it all morning. Yes I do know why we never see a free floated military barrel (except with todays application for SDM and sniper rifles). Bass, free floating barrels wasn't discovered (and accepted) to be a real aid to accuracy until the late '50s after the bench rest crowd proved over and over again that free floated barrels are more accurate. All of the surplus military rifles we have and shoot were made well before that discovery. Also all of the factory rifles were made with barrel pressure as that was the accepted way then. Those rifles were bedded with the techology that was known when they were made. That's WHY we don't see any free floated barrels military barrels.

You said yourself, jacketed accuracy can be found at low, medium or high velocity. Jacketed throws enough friction that as the speed increases, the vibration speed increases and the barrel whip is minimized. No bedding is needed because you simply alter your powder charge or your powder speed to get the vibration pattern needed for consistent shot to shot exits.

The wine glass with your finger routine. Slow and easy and the pitch is low and soft. Push harder and the pitch of the tone gets higher and louder. Ever play the geetar? A real lose string can be seen vibrating and drawn tight never seems to move.

With lead, you must stay with the slowest powder producing your best velocity or accept a lower velocity (RPM).

No problem with all of that, we're on the same page of the hymn book there.

So jacketed accuracy is a different bedding game. Stop here. You have to believe this or your wasting your time reading on. Lead bedding logic SHOULD be different. It doesn't "have to be" if you can get best accuracy within the harmonics range you want to operate in with the barrel you have. (HV 30 caliber seems to be 2200 to 2400 fps) Bedding won't help then if you are accurate and it could make things worse. But lead, you can only throw so much friction before your lube breaks down and you lead. SO if your best (most accurate) load effort CAN"T reach the proper harmonics, then you MUST change the oscillating frequency which may be more pressure or more pressure farther out. Bedding does this. The farther out vibration is controlled, the less pressure is needed. But if you don't tune your lead, you live with the result.

Guess I'm wasting my time but that's alright. You see all my rifles are not free floated. I have numerous military rifles that are still bedded as made. I do mostly bed all my sporters, varmint and target rifles. I've numerous times tried adding pressure to barrels to enhance accuracy. Only in a few instances over the years has it worked. That was most often with very thin barrels and the long (29 1/2") Mauser barrels when those were in sporter stocks. With barrels of normal contours they ALWAYS are most accurate with jacketed and cast bullets.

Now I'll give you a case in point. Years back I aquired a new 2 groove '03 barrel with 10" twist. Now everyone and their brother swore that 2 groove barrels are more accurate with cast bullets (they aren't by the way) so I thought I had the cat's meow for a cast bullet rifle barrel. I also had a M1916 (Oviedo made in '28) Spanish M93 action that had just shot out it's 3rd .308 Winchester barrel. I had converted it to cock on opening so the lock time was very quick. It had been a very accurate rifle with all 3 previous barrels shooting both J and cast bullets. I had dreamed up my .308 CBC then which is a shortened .308 case with an '06 length neck. case capacity was computed to be just enough 4895 to get a 311284 to 2200 fps. I had the 2 groove barrel set back, threaded for the SR mauser and short chambered with a tight necked .308 reamer and then a '06 reamer was used to lentgthen the neck. Conversion came out quite nice. Case were formed from unfired U.S. 7.62 cases with the necks turned for .002 clearence with a .311 bullet. However I quickly found that accuracy was best (1- 1/1/2 moa) with 311284 in the 1750 to 1850 fps range which is right smack dab in the RPM threshold. I was not yet aware of the RPM threshold as i didn't read about it until some time later. When I increased velocity, thus increasing RPM, accuracy deteriorated to 3-5 moa, That's exacty what that bullet (or 311041 or 311291) did in my 30-06 and one .308 which also had a 10" twist. To the point here; I have tried every concievable thing, including all manner of pressure to the barrel and nothing changes the accuracy. Yes , I also went to slower powders but the smaller case capacity didn't help. Point is, pressure to the barrel changed nothing as far as accuracy at any velocity when using cast bullets. I have also tried barrel pressure numerous times over the years on many different rifles. This was even recently with the RCBS 30-150-FN after I shot your 154 LBT bullets. Pressure of 2, 6 and 10 lbs did not change the accuracy one iota with those cast bullets at velocities from 1800 up through 2500 fps. Accuracy was still the best (1 1/2 moa consistantly with 10 shot groups) down in the RPM threshold and went to 5-6 moa (with 5 shot groups) at the high end.

(It's the same with jacketed and thus the 115 grin Speer reference in the 8.66 twist. While people believe it's the twist rate overstabilizing, it's the increased barrel FREQUENCY from speed (the finger on the wine glass again) making bullet harmonics and bullet exit at a single point (accuracy) even MORE difficult. We prooved in several rifles that light bullets COULD BE shot MORE accurately than heavy for which the 8.66 twist was designed. All we did was change bedding.)

I get quite good accuracy with that Speer bullet out of my M95 Mauser. I also know of several other 7mms that shoot that bullet well. Would appear more to me that you have a barrel there with a lot of internal stress. What most call a bad barrel. We still agree that pressure was helpfull there.

Now, when a lead bullet reaches the point that lube breaks down and it .... grabs, that barrel and the vibration pattern is changed dramatically. (You pushed your finger harder on the wine glass and got a louder sound) So you get a flier. If you bed so that that change is trivialized, then you get less of a flier. Or so that is my logic. Same thing for minimizing humidy effect on lube and temperature, and metal fouling too. This is why militaries grab their barrel and hold it so as fouling alters accuracy, the effect is trivialized. A benchrester simply cleans his gun. Bedding does externally what a powder or charge change does. It's as simple as that. Maybe not as well, but well enough for lead.

I pretty much agree with here but I'me not sure how you think the defects caused to the bullets aren't affected by the rotaional spin of the centrifical force of the RPM when the bullet leaves the barrel. That is what we're talking about here. That is where accuracy is primarily lost; the defects in the bullets (from what ever reason) are accentuated by the increased rotational speed at higher velocity and accuracy is not as good as it is at lower RPM. If a lube breaks down and the barrel is leading I don't really care what direction the barrel whip throws the bullet it is not going to be accurate. Have you ever had a leaded barrel shoot accurately? I haven't.

Works as long as you aren't destroying bullet because of fouling, then all bets are off. You have to understand once you reached that point. Based upon my 35 Whelen tests last month with 4-6 BHN harder bullets and how that affect accuracy, I am going to get a .308 sizer and harden my bullets a tad and see if I can't shoot HV this winter.

I think you're forgetting the adverse effect of obturation at high accelleration. Even if the lube doesn't break down and the fouling is minimal adverse obturation if cause by higher acceleration will cause imbalances in the bullet and accuracy will be adversely affected by the higher RPM. I will be most interested in the results of your tests. I too will test this winter. I will run identical loads through 3 very accurate .308 rifles; a 10" twist M788, A 12" twist M70 Target and the 14" twist M98 Palma rifle. I will keep you posted.

When you slow the twist rate, you are in effect cutting pressure that alters the vibration pattern to allow higher velocity (RPM)s before the lead becomes unstable changing vibration again. Why not bed?

I've seen numerous tests that show the pressure between a 10" and 12" twist barrel is essentially the same. Any measured difference fell between the shot to shot variation of pressure. Not sure how this is with a 14" twist though. I am probably going to put my money where my mouth is and get a piezo (SP?) pressure measuring system for Christmas. Hopefully I'll have an answer by next Summer.

One more point: Larry "So what's that prove? Nothing much except your bullet shoots much more accurately down in the RPM threshold. That is just what I have been saying."

No all that prooves is that your gun shoots that bullet more accurately at that velocity. Free floated aren't ya?

No Bass, they are not all free floated as previously stated. With or without free floated barrels the best accuracy with each, when using cast bullets, comes within or under the RPM threshold.

To be continued folks....

Larry Gibson

Larry Gibson

11-29-2007, 12:48 PM

joeb33050

Not ignoring you, just haven't had time to check the threads. Thanks for the reply and I do not doubt your veracity. I'm just always intersted in learning. I'll get back with a reply.

Larry Gibson

Bret4207

11-29-2007, 01:05 PM

joeb33050

11-29-2007, 02:38 PM

So do you guys think a boolit (cast alloy) can actually be spun fast enough to cause the alloy to "fly apart"? Is it even possible for an alloy to resond like that or would it just become radically unstable and turn into a tumbling rock? Is the "vaporizing bullet" , either cast or jacketed, another old wives tale?

This is a topic that JA is working on. I asked another engineer here to handle it, but evidently I offended him-called him a name-in the past. This is easy, since:
There's no such thing as centrifugal force.
There is centripetal force.
We know the tensile strength of lead and alloys.

I suspect and forecast that no lead / alloy bullet will ever come apart because of the rotation.
I suspect that bullets blow up because the jacket or side is displaced or cut, amking the bullet wildly unstable.

Any competent engineer should be able to answer this question in a short time.
joe b.

joeb33050

11-29-2007, 02:42 PM

Here's a summary of RPM for CBA 2006, 2007 NM, with unknown and SS Breech seaters out.
Attached is the workbook. No it's not, this BD thing (B stands for BROKE) won't accept .xls files.

2006 CBA NM
RPM
Average 125,578
>150,000 1
140,000-150,000 6
130,000-140,000 3
120,000-130,000 10
110,000-120,000 9
100,000-110,000 4
<100,000 1

2007 CBA NM
RPM
Average 132,246
>150,000 2
140,000-150,000 6
130,000-140,000 12
120,000-130,000 6
110,000-120,000 7
100,000-110,000 9
<100,000 2

11-29-2007, 02:48 PM

Although I've been keeping quiet, this thread has held my interest. It's my personal belief that there are more variables than we are willing/able to control simultaneously in this endevor. Thus the never ending discussion and room for "voodoo" in the process. To get real empirical evidence would require about ten identical rifles, each of which was bedded, rebedded and/or re-barreled simultaneously throughout the process.

I'm speaking up at this point to say that the "gray dust" phenomenon is possible, and I have witnessed it. I watched a dozen 40 grain .224 varmint bullets disappear with the evening sun at our backs out of a 9 twist .220 swift. These were loads that pushed a little better than 4,000 fps out of a 14 twist barrel. The 9 twist was built by a friend for sending the heavier bullets to coyotes across the frozen, windy lake. The 40 grainers were left from the previous 14 twist barrel and shot specifically to see if they'd go to pieces. They did. You could see the dust streaks as well as the fragments, (jacket bits?), kicking up dust downrange.

BD

11-29-2007, 03:00 PM

I'm not claiming that I know what specific factor(s) causes the gray dust, only that I have seen the effect.

I'm of the opinion that sectional density may play a role as well. This is based on the single observation that if I push the 140 grain 6.5 Jumptrap's to 2,000 fps in a 7.8 twist Swede the groups open up to 14" or 16". If I push the cruise missle to the same velocity in the same Swede the groups open up to 6' or 8'.

BD

Chargar

11-29-2007, 03:01 PM

Felix.... I am certainly glad the tech folks came up with a term/process to deal with the crap we simple minded folks find stupifying.

Larry... Yea... I sorta figured the given the same peak pressure, a slow powder would allow more RPMs than a fast powder.

I think we have known for several lifetimes that with fast powders accuracy goes South at a lower velocity than with a slower powder. We always knew it was how fast the pressure was applied to the bullet that resulted in the accuracy differential. You boys are just trying to figure out why?

Blammer

11-29-2007, 03:12 PM

so, out of all of this....

I should take that a GOOD place to start with a .308 dia bullet of 225gr (311284) for a 30-06 with a barrel twist of 1-10 would be to try for 1800 fps to start for a good accurate hunting load?

Junior1942

11-29-2007, 03:23 PM

Larry Gibson

11-29-2007, 04:49 PM

waksupi

11-29-2007, 06:05 PM

Guys, I'm not the brightest bulb on the Christmas tree, when it comes to math.

Is

(velocityX60) divided by (Twist divided by 12) = RPM the correct formula?

Nueces

11-29-2007, 06:16 PM

Yup, that's it, or, the same thing; 720 (which is your 60 times 12) times velocity divided by twist.

Mark

KYCaster

11-29-2007, 09:07 PM

Velocity in ft/sec X 60 = velocity in ft/min X 12 = velocity in in/min, divided by twist = RPM.

Jerry

andrew375

11-30-2007, 02:21 AM

Andrew375

That is a nice group, especially for a belly shoot. The velocity is chronographed? As you say in that thread that one group does not accuracy make so does it do that well consistantly? You obviously have a nice load there at any rate.

Larry Gibson

All my loads are chronographed. I don't make any claims as to consistency as there are too many variables muddying the water introduced by the shooter. The rifle may be consistent but I'm not! I am getting better though.

joeb33050

11-30-2007, 05:23 AM

Again, if the RPM threshold is based on what folks are shooting, then I think we're making a mistake.
The Production class revolves around a Savage .308 with a 10" twist and a bullet going 1800-2100 fps. This isn't because folks choose the gun or the twist or the cartridge, it's because that's what's out there and available.
If we had our choice, the Savage might be in 30BR or 7.62 X?? or 30/30-smaller cartridge case; and the twist might be 12 or bigger #.
One 2007 competitor used a 18" twist barrel in 30BR at 2750 fps with a 108 grain monotype bullet.
The CBA experimenters/rifle builders seem to be going toward the 30BR Score jacketed direction without-maybe-thinking this through.
The SS breech seaters are shooting almost as well at much lower velocities with heavier bullets. Why aren't the experimenters going toward long heavy bullets in gain twist barrels at 1400 fps?
joe b.

felix

11-30-2007, 07:15 AM

JudgeBAC

11-30-2007, 07:45 AM

This may be off topic and, if so, I apologize. Is the following observation a factor of velocity, RPM, a combination of the two, or some other factor? My model 29-10 6.5" bbl with a 255 gr. Keith bullet over 9 gr. of Unique places these 1" above the sights at 25 yards and to a similar place at 50 yards. A 277 gr. MM bullet over 9 gr of Unique places the bullets 3.5 to 4" LEFT of the sights at the same distances. What gives?

felix

11-30-2007, 07:56 AM

Position of impact at 25 yards with a revolter has more to do with how the gun recoils more than anything. The left-right business at that distance might have something to do with how the boolit style makes it through the forcing cone based upon mechanical conditions. I have found that the Unique speed of powder needs to be very close to being right on. If you vary the powder by half grain on either side of a proverbial sweet spot, and the accuracy goes out by a visual amount, then pick another powder. So, try to adjust the loads between each boolit. This is more of a question for the 44man and others who play hard ball with pistols. ... felix

JudgeBAC

11-30-2007, 08:11 AM

Felix, thanks for your thoughts. I moved this query to the wheelgun forum.

Larry Gibson

11-30-2007, 09:21 AM

All my loads are chronographed. I don't make any claims as to consistency as there are too many variables muddying the water introduced by the shooter. The rifle may be consistent but I'm not! I am getting better though.

Understand, I've shot quite a few "belly" matches myself.....some days are diamonds, some are stones!

Larry Gibson

joeb33050

11-30-2007, 09:25 AM

Reason: because the longer the boolit is, the more distortion it receives in a gain twist barrel. If the barrel is going to be made with cut grooves, then a very, very small amount of gain twist might help the cutter mechanism do a more consistent cut job. ... felix

I don't think so. The bullet is going to turn and get engraved no matter what.
Smith gain twist barrels are used by the SS guys with good results.
I think the CBA fixed experimenters are like all people, doing what's new and popular without stopping to think about it deeply.
Gain twist has worked for 150 years, although a good fixed twist barrel works fine too.
joe b.

felix

11-30-2007, 10:07 AM

There is a difference in knowing so and thinking so, Joe. However, on your behalf, there is safety in numbers. ... felix

Larry Gibson

11-30-2007, 02:08 PM

Woke up this morning and since the wife was sleeping in I turned on the Mag 20, put some 60/40 WW/Lino alloy in it, degreased two Lyman 2 cavity moulds (311291 and 311466), put a pot of coffee on, turned on some smooth jazz on the radio and commenced to cast 16 lbs worth of bullets.

My plan is to test identical loads for for three .308 rifles; a M788 with 10" twist, a M70 target with 12" twist and the M98 Palma with the 14" twist. The goal is to observe accuracy improvement or deterioration as velocity is increased from around 1700 fps up through 2500+ fps with each rifle. The reason for the test is to observe what the average guy may get accuracy wise with the .308 based on different twists. I chose the 311291 because it is readily available and a proven design. The 311466 was chosen because it is Lymans' fastest .308 Winchester bullet (2920 fps with 748) in their new Cast bullet Manual.

Bullet selection will be by visual inspection for defects with only the bullets with complete fillout and no wrinkles or voids being used. I do not intend to weight the bullets or sort them by cavity. Right now the 311291s are .309 - .310 so it looks like a .310 sizer will be used. The 311466 drops at .315!!!! Not sure whether I'll test them at .314 or size them to .311 (after lubing) for the test. Gas checks will be Hornady's and they'll be seated using the GC seater on the 450 then the 311291s will be run through a Lee .310 sizer then lubed in a Lyman .310 H die. The 311466's, after seating the GCs on the 450, will be run through a Lee .314 then lubed in a Lyman .314 H die in the 450. It may be shot as is or then run through a .311 Lee sizer.

Cases will be once fired (through the M70) LC 92. Necks will be inside reamed in a Lee Target Loader for unifromity and concentricity. Primers will be WLR and Remington 9 1/2s. Lube will be Javelina as I've not had any lube failure with it up through 2600+ fps.

Powders will be; 4895 with a Dacron filler used until loading density is 80%+, RL19 and H4831SC.

Testing will be done at Tacoma Rifle and revolver Club which has very sturdy cement benches. All loads will be over the Oehler 35P chronograph. Targets will be at 100 yards. The end result will be NOT to compare the accuracy of the rifles against each other (I already know which rifle will win that one). The test will be to observe accuracy of each rifle as it increases or decreases through increasing powder charges and how accuracy responds as velocity/RPM is increased. Testing identical loads in the 3 rifles with 3 different twists should then give us some indication on the effects of RPM.

Any other suggests? Remember this test is to replicate those cast bullet loads that the average cast bullet shooter would produce. As the average cast bullet shooter doesn't get too anal in reloading technique/detail neither shall I.

Larry Gibson

felix

11-30-2007, 02:35 PM

Good show, Larry! Should be a fun experiment. No hurry. ... felix

sundog

11-30-2007, 02:51 PM

Hooah, Larry. Over a chrono, but what are you gonna do? Groups for each load for each rifle?

My take is that your test will actually produce enough data to tell something. I'll reserve my SWAG until after your data is compiled (although I have my suspicions). Looks like you'll be getting some quality trigger time!

Blammer

11-30-2007, 03:32 PM

got any 311284 boolits? Them would be nice to test too.... lol as if you don't have enough covered.

Pat I.

11-30-2007, 05:17 PM

I think the CBA fixed experimenters are like all people, doing what's new and popular without stopping to think about it deeply.
Gain twist has worked for 150 years, although a good fixed twist barrel works fine too.
joe b.

Of course this is right. I spent 250 bucks on a 17 twist Shilen barrel, 150 on a reamer, 125 for a LBT mould plus the RCBS 22 cal mould blocks I used to make another bullet, brass, dies, powder, time putting it all together, and a bunch of other things and did it all on a whim with no thought involved. Since there was only my 17 twist and the other fellas 18 twist, which you failed to mention he used to win the unrestrcted class with, 30 caliber barrels on the line I don't think either of us chose it because it's new or popular. His is a 30 BR mine is a 30x47, lots of those being shot isn't there.

waksupi

11-30-2007, 07:07 PM

Harry Pope, and his contemporaries, never did really settle on wheter gain twist was better or not. I'm with them.

ssn vet

11-30-2007, 07:29 PM

this is really interesting......especially for newbs like me....

reminds me of the velocity limits published by the plated bullet mfg.s

Berrys normally states a limit of 1,200 fps on their pistol bullets and on the 150 gr .308 FN I load for my Marlin 336, they say 1,700 fps (numbers pulled from memory, it's been a while).

The best explanation I've heard for this is because the copper plating starts to "spin off".....but I wonder

These plated bullets have a pretty soft swaged cores, I wonder if that comes into play....

sorry for the thread hijak, I find the topic very intereting

felix

11-30-2007, 07:52 PM

No hijack... Yes, a heavy (and not too tightly woven) core will want to push off a jacket. Any hole found will let loose a bomb, re 220 swift, 22-250 types using the thinner jacket bullets. Maybe a depleted uranium core will take 2 times that rotational velocity before even thinking about getting nervous. ... felix

joeb33050

12-01-2007, 04:40 AM

Of course this is right. I spent 250 bucks on a 17 twist Shilen barrel, 150 on a reamer, 125 for a LBT mould plus the RCBS 22 cal mould blocks I used to make another bullet, brass, dies, powder, time putting it all together, and a bunch of other things and did it all on a whim with no thought involved. Since there was only my 17 twist and the other fellas 18 twist, which you failed to mention he used to win the unrestrcted class with, 30 caliber barrels on the line I don't think either of us chose it because it's new or popular. His is a 30 BR mine is a 30x47, lots of those being shot isn't there.

Pat;
You seem defensive. There's no need. Maybe it's a coincidence, but the trend to slow twist short bullet rifles in CBA seems-seems-to follow that trend in jacketed score shooting.
Comparison of results in CBA NM shooting has shown the PB breech seaters doing virtually as well as anyone else, when there are enough of them attending.
Perhaps the slow twist short bullet fast plan is the route to ultimate accuracy with CBs, but I wonder what could be done with long bullet appropriate twist slow bullets with ?gain? twist barrels.
You've got the "where we are now", then two diametrically opposed alternatives that present themselves. Movement seems to be toward one of those alternatives, with no movement toward the other.
Maybe, as I said, maybe, this is happening without the experimenters thinking this through all the way.
And maybe not.
Regardless of that, my impression is that the slow twist fast short bullet guys do not go to the slow twist because they percieve an RPM threshold ahead.
joe b.

joeb33050

12-01-2007, 04:44 AM

Harry Pope, and his contemporaries, never did really settle on wheter gain twist was better or not. I'm with them.

I certainly can't prove it, but the ASSRA scores/groups improvements in the past few years might just have something to do with those Smith (and other) gain twist barrels.
Something happened, so that we see multiple 250s shot in matches.
joe b.

felix

12-01-2007, 07:40 AM

joeb33050

12-01-2007, 08:38 AM

Again, Joe, shorter the boolit (engraving section) , the better the gain twist works. Also, the tougher the boolit, the better it works. Gain twist requires that the boolit comes back to normal after changing from being twisted at different rates from front to back within the engraving section. .... felix

Shorter = works better. Any data?
Tougher = better. Any data? Bev Pinney, PBB winner, used 30:1, Smith gain twist. Duane Jenner used 25:1, Smith gain twist. These were the two gain twist guys at the 2007 CBA NM

I don't know what your last sentence means.
If the gain twist gains at all points along length, then the front of the bullet is being twisted faster than tha rear, but imperceptibly in any normal = 1" bullet length. There is always this, and always the angular delta per inch of barrel increases.
Starting with the bullet breech seated, the front of the bullet has been turned at some angle theta and the rear has been turned at theta-D for difference.
As the bullet moves down the barrel, twist increases, theta increases, and theta-D increases.
With a gain twist barrel, the first derivitive of theta or theta - D WRT L = barrel length, is a positive number. With a constant twist barrel the first derivitive is zero.
Pope wrote that the constantly changing angle kept the bore-bullet interface sealed. Or sealed better. This of course was before the invention of obturation.
I don't know if the gain twist helps accuracy, results are mixed.
I do think that in a good bolt gun, a breech seated bullet may shoot better, and it may shoot even better in a gain twist barrel.
And, the gun is available, unlike the DeHaas Miller actions that are as close to unobtainable as possible.
A stock Savage 308 with breech seated bullets is easy to try, and the twist is fast enough for long bullets. A breech seater can be ginned up by a competent machinist or hobbyist.
If that worked, changing the gun to switch-barrel and using a ?Smith gain twist barrel in 30BR or a cartridge similar to 32MS in volume and shape would allow a first-look estimate of the effect.
The Savage is approximately infinitely more available than any competitive SS rifle, and 1/4 to 1/6 the cost, and years faster to acquire.
At my age I have to consider the gunsmith velocity if I'm having a gun built. Answers to the analysis are not encouraging. I may get the gun before I croak, but will I know why I ordered it and what to do with it? Or my name?
Anyhow, why doesn't someone tell me where I can read about the Government testing having to do with RPM???
Larry? Anybody???
joe b.

Pat I.

12-01-2007, 11:49 AM

Larry Gibson

12-01-2007, 04:13 PM

joeb33050

The tests were conducted by the U.S. Ordnance Technical Committee. They recommended the adaptation of the T65 cartridge (7.62 NATO) in a rifle with 1-12" twist. I'm not sure where to find their published report on it as it's been years since I read it. It is most often erroneously published that the T65 cartridge was to replace the M2 cartridge with it's attendant velocity of 2800 fps. That is not the case. First of all the Arsenals did not produce any M2 ammo that attained 2800 fps. That was the plan but the M1 cartridge superseded the plan to up the M1903 ammo velocity to 2800 fps (originally 2700 fps). Problem was the M1 ammo’s range exceeded the maximum the range fans of most military ranges allowed. M2 ammo came about when the arsenals were asked by the War Department to load some equivalent ammo to M1903 ammo for “practice” stateside as the WWI produced stocks of M1903 ammo had been exhausted. M2 Ammo was loaded to nominal 2650fps by the arsenals to meet this requirement. I’ve a copy of the memo directing such velocity BTW.

Back to the point; T65 ammo was designed and loaded to match the trajectory of M1 ammo (174 gr FMJBT at 2640 fps) out to 1200 yards (1100 meters). The 147 gr M80 or the M59 150.5 gr bullets were loaded to 2750 fps 78 ft from the muzzle. It was found a 12” twist gave the best accuracy at maximum range over a 10” twist. Note: many also think the max effective range of the M14 was/is 450 meters (500) yards but that also is not correct. The 450 max effective range is the max effective range using hold over with a battle sight zero (250 meters). Maximum effect range actually is 620 meters for the M14 and 1100 meters for the M60 GPMG. With the rear sight of the M14 properly zeroed it is quite possible to adjust the sight (that’s what the elevation scale is for on the elevation knob) and hit an E target 80% of the time at 700 yards (the 620 meters).

Palma Matches were originally shot with issue rifles and ammo provided by the host country. Palma Matches are belly matches fired at 800, 900 and 1,000 yards. Since 7.62 NATO was the ammo in use by the host nations the ball ammo of 145-155 gr was used. The rules changed that allowed the visiting nations to bring their own rifles, initially service rifles and then any rifle, in 7.62 NATO as the host nation continued to supply issue service ammo. The match shooters picked up on slower twists not accentuating the defects due to RPMs. Initially the match rifles had 12” twists but then they lengthened the barrels to 27 ½ - 32” to obtain maximum velocity. This increased RPM and accuracy with ball 7.62 ammo was adversely affected. It was quickly found that a twist of 13 or 14” dramatically improved the accuracy of 7.62 Ball. Some equivocate this with rounds that “go to sleep” but it actually is different. The adverse affects that RPM has on bullet imbalances is a proven fact.

Larry Gibson

Bass Ackward

12-01-2007, 06:13 PM

The adverse affects that RPM has on bullet imbalances is a proven fact. Larry Gibson

Twist rate is determined by the range that you want to fling a certain weight / length / ballistic coefficient object in a stabilized fashion. The more screwed up or less aerodynamic the projectile, (the wider the meplat) the more velocity you will need to start it out at. If your velocity ceiling is limited, then you will need to increase your twist rate to stabilize it farther out. (wadcutters) This is external ballistics and why nose shape / meplat size eats up twist rate. Or lowers RPM ceilings.

Internal cast ballistics dictates that a slug trying to accelerate is retarded by a twist rate angle in the bore. The more the rate angle is increased, (faster twist) the more that bullet will be held back and pressure will work on the base. In effect, it's the same as using a faster powder and we all know that slower powders do allow higher velocities. :grin: Also the more barrel time it will have exposed to that pressure. And the result will be the more barrel vibration and greater harmonics will take place. That is also why the lightest bullet per caliber achieves the highest RPM. And why softer bullets can be used with slower twist rates. And why larger case capacities lengthen pressure curves. Not because of stripping or bullet damage as most believe.

So, when you lose cast accuracy is it because of external ballistics or internal ballistics at short ranges? :grin: Actually both. You work to mold to achieve perfection and your twist rate fights to destroy it. Pressure does the damage.

Larry Gibson

12-01-2007, 06:51 PM

Bass

It would it appear that you agree with the statement of mine you quoted! I've no arguement with your discertation as it quantifies the quote.

Larry Gibson

sundog

12-01-2007, 07:40 PM

Gain twist is just that. BUT, you really need to know the start and end twist... 10 to 12? 13 to 14? AND what bore? .308ish, 32ish, ??? Otherwise it doesn't mean much. Gradual, abrupt?

Anyone know for sure the start/end for the Smith bbl?

I shot today in the big wind. S20-G40, light rain showers, and VERY overcast.

Chargar's Lee 311291 in the .303 over 18.0/surp4759 and dacron wad went less than half inch by 4 inch vertical for ten shots. GT line is 360 (wind directly at back). Thinking about bumping it a grain.

Also shot 03A3 scoped sporter with 25.0/4198 and dacron wad with Beagled SAECO 301 WQ, Beagled 311284 WQ, and 314299 HT. 311284 was purdy good, but as usual the 314299 was the best. It and the 311299/40.0/4895 WQ can hold the white of the old gieser target at a hunert for five shots. Worth the trip to the range.

sundog

12-01-2007, 07:41 PM

Oooo,, actually a gain twist would be something like 14 to 12, number has to reduce to speed up the spin....

Sorry 'bout that.

Pat I.

12-02-2007, 05:04 AM

I won't bet my life on it but I think you can choose the start and end twist with the Smith barrels, like 19 to 11 or 20 to 14. If I remember right the Carcano's was something like 19 to 8 and would be a cheap way for Joe to test it out.

joeb33050

12-02-2007, 05:28 AM

How do you know Bev or Duane are using gain twists? I don't and I know them both. Why don't you find a 6.5 Carcano to play with? If you decide instead to have a Smith GT barrel put on one of your guns, it doesn't have to be a Savage to see the results, keep me posted since I'll be wallowing around doing things without thinking them through.

"How do you know Bev or Duane are using gain twists? I don't and I know them both."

I consulted the Nov Dec 2007 Fouling Shot, looked in the NM equipment lists, and, as they say, voila!!

I think I'll try breech seating a bolt gun, I mentioned the gunsmith velocity which keeps me from trying to have a gun built. The famous Competitor SS pistol that was to be delivered in under 10 weeks is now 17 weeks underconstruction and no news.

joe b.

Pat I.

12-02-2007, 06:02 AM

[QUOTE=joeb33050;252629 I consulted the Nov Dec 2007 Fouling Shot, looked in the NM equipment lists, and, as they say, voila!! joe b.[/QUOTE]

Well ya got me there for sure! Guess it gives credence to the old saying that even a blind hog can find an acorn once in a while.

Bass Ackward

12-02-2007, 06:59 AM

Bass

It would it appear that you agree with the statement of mine you quoted! I've no arguement with your discertation as it quantifies the quote.

Larry Gibson

Larry,

Not always. Remember, the 115 grain Speer example. If something can be beaten just once, then it isn't a fact. A better term might be "inflexible trend". The real question is how flexible (anal) do you want to be?

What's the most accurate cartridge in the world? A 30 PPC at 100 yards? A 300 Win Mag at 1000? Or a 303 British at 1319 meters? (I think that distance is correct.) Can a palma rifle compete to win in any of these categories with generic bulk ammunition regardless of twist rate? Who expects to do well at a 1000 yard match with a 20" barrelled 30 PPC? Or someone at a bench rest competition with a military 303. So why can't we consider a cast as another possibility to make use of our desires instead of using only a set amount of variables to draw up .... "easy" limits?

So why when we come to lead, is the limit the lead because of a single variable? RPMs? Don't we match hardness to the pressure to defeat it? And if going up in hardness by just 6 BHN requires .001 down sizing in bullet diameter for me to get the same groups, can I not be anal (sometimes you don't have too as some guys can find HV almost by accident.) to try something else instead of just blaming it on RPMs?

In other words, if just one guy can, ask why can't " I "? The answers are advancing. If you accept an RPM level as "your" guide, you trivialize the shooting and reloading process to blame your or your guns failure. Simple as that. So all of this jumps around the hot button issues. We don't define accuracy standards. We don't define poor or defective cast guns. And we don't define anal. And without that, we can't develop an RPM ceiling without including .... defective and limiting data. (statistics)

So your test to find best "average accuracy" with what's "reasonably" available off the shelf or steps that are taken is fairly useless. If you want to prove "easiest accuracy" that a slower twist is more .... "idiot proof" and minimizes errors as a reloader and shooter of cast, save your components, I'll give you that. Cast or jacketed as your palma post shows.

Question that should be asked, are you interested in generic accuracy or specialized and specific accuracy? One requires more than the other. But just because you choose to run your two mile PT test with a 35# ruck on your back filled with beer for the trip, don't blame me when your times don't meet the standard. And that's what your test is really talking here. I want this without having to do or meet that. Fortunately, cast accommodates everyone and the HV guys are changing the stereotype. :grin:

joeb33050

12-02-2007, 10:34 AM