RPM Threshold

[Bass Ackward]

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.

[leftiye]

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]

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]

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]

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.

[Bret4207]

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]

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]

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.

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]

[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]

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.

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

What powder? 4831?

[felix]

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]

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.

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

Larry Gibson

[Larry Gibson]

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]

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]

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]

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]

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]

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

[Bass Ackward]

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]

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