RPM and Bore Size

[Johnw...ski]

Still trying to get a handle on this RPM thing.

Does bore size have an effect on what the optimal RPM will be?
Trying to make some sense of RPM's and also the Greenhills Formula
and real life without success.

I have a .458 barrel with 1 in 14" twist, I am useing a lead bullet 1.05 long.

Using the Greenhills Formula 150 x .458 x .458 / 1.05 = 29.97
Seems to me to be a rather unusual twist rate.
Carrying it further the optimal bullet length for my 1 in 14" twist comes in at 2.25"
a pretty long bullet. When I calculate smaller calibers the twist rates seem much more reasonable.

Now on to RPM, it seems to me that if there were an optimal RPM or RPM range the twist would always come out the same given the limits on cast bullet velocity.
What I am saying is, if your target velocity is say 1600 FPS and your target RPM
is around 82,000 RPM (720 x 1600 / 14 = 82,286) then a 14" twist would be about right no matter what the bore size.

Anyone have any target RPM's for different calibers and velocities?

It seem that lead bullets have a rather limited velocity range compared to jacketed bullets so the twist rates should be rather close, or at least closer than Greenhills suggests.

Confused,

John

[Larry Gibson]

With a 14" twist the RPM threshold runs from about 2400 fps to 2700 fps. With larger bullets like .458s usually the shooter is "recoiled" out before 2400 fps. My 450-400-70 Siamese Mauser barrel has an 18" twist barrel. The fastest cast bullet I have shot in it is a 385 gr 457483 GC cast of linotype. I pushed it to 2350 fps and while that was only about 94,000 RPM it was all the fun I could stand from the 8 1/2 lb rifle off the bench. Quite frankly the first 3 shots were good , accurate also at 2" but after 4 more attempts to get a shot off without flinching I quit.

Did I ever tell you guys about the day I was at the range and the ghost of Rocky Marciano showed up? Every time I pulled the trigger I took a wicked left hook to the jaw followed by a right to the shoulder. I was "out" in 7 rounds (pun intended)!

Larry Gibson

[JIMinPHX]

So then, what would the RPM threshold be for other twists, like 1:9, 1:10 & 1:12?

[leftiye]

Johnwski has a muy good point. The bigger the diameter of a boolit/bore, the more centrifugal force is generated at a given RPM, and this is transferred to the defects on a boolit and results in more deflection. The RPM barrier ought to be lower the higher the caliber. What stops this from showing up is the fact that in .45 caliber the boolits are proportionately shorter per caliber, and therefore more stable inherently.

Which inversely should result in higher RPM barriers for shorter boolits???

[Larry Gibson]

So then, what would the RPM threshold be for other twists, like 1:9, 1:10 & 1:12?

The RPM threshold where best cast bullet accuracy will be with a medium to slow burning powders.

Roughly; I say "roughly" because I compute RPM the old way in lieu of the formula used above and siply because if you've too soft an alloy accuracy will deteriorate before the threshold and conversly with some designs, the right alloy and the right slow powder you can crowd the top end of the threshold. However with your basic WW or #2 type alloy of BHN 14-18 and a regular design GC cast bullet the threshold is what it is...a threshold. BTW; the RPM threshold is that point where the RPM adverely effects the imbalances of the bullet and overcomes the rotaional stability to the degree that accuracy worsens as velocity and RPM is increased. A "threshold" is different than a limit or boundary. The threshold can be crossed but one is not going to do it with regular cast bullet designs (expecially those with bore riding noses) or with softer alloys.

1:9....1525-1750 fps
1:10...1725-1950 fps
1:12....1900-2300 fps

Larry Gibson

[felix]

Leftiye, RPM threshold goes up with smaller diameter boolits as you reasoned. Conversly, the longer the boolit the lower the threshold. ... felix

[JIMinPHX]

So then, I'm doing well to have been getting good groups from a 1:9 .223 @2000fps with a 13-bnh 55-grain gas checked flat point?

Correction –
I was told that the barrel was a 1:9. I just checked it. It’s a 1:12.

[Johnw...ski]

Some thoughtful answers here. I would love to hear from some long range shooters and benchrest shooters using lead bullets. What velocity, twist , caliber works for you?

The rules between jacketed and lead are very different.

John

[Bass Ackward]

Rifling height also determines RPM threshold if you want to call it that. The lower the rifling height you have, the lower your threshold will be before you go screwy. Get it low enough and you will have to shoot rock hard at 1600 - 1800 fps what RPMers refer to as the ideal operating range.

So smaller diameters have a larger percentage of the bullet diameter engraved with .004 tall rifling than a 45 caliber with .004 tall. Luckily, you can use a slower twist rate there cause twist rate is an impedimate to forward motion. That's why using the same hardness bullet, the slower twist rate will always have the higher velocity potential.

Wider drive bands and bullet designs with less nose length will have higher RPM potential too with the highest being the lightest bullet percaliber cause it will have the least pressure on the base and the longest pressure curve.

The RPM theory is a "history" of what can be expected from all guns, even the poor ones. All bullet designs, even the weak ones. All hardnesses, even the soft ones. All standard twist rates, using all standard lead thought processes over time.

That's where thinking in terms of RPMs is going to always put you at a disadvantage if you ever want to leave the .... zone. Then when you do, you will add your results (failure) to the history reinforcing the theory already established. You will never see the true causes to be able to work around it.

[Bret4207]

"Rifling height also determines RPM threshold if you want to call it that. The lower the rifling height you have, the lower your threshold will be before you go screwy. Get it low enough and you will have to shoot rock hard at 1600 - 1800 fps what RPMers refer to as the ideal operating range."

Interesting thought Bass. I think you've mentioned it before but this time it tickled a thought. The Steyr 8X56R has rifling about a mile deep, at least mine does. I'm trying for some 338 moulds right now and have the 338 Lee mould. I wonder if that deep rifling would allow a full groove diameter boolit to work at high velocity than lower rifling? Wish I had the time to try all this stuff.

[pdawg_shooter]

I have a 458WM built on a P14 action with a Douglas 1/22 barrel. I built it to explore the limits of paper patched bullets. I have taken Linotype to 2612 with good accuracy. Bullets with a BHN of 7 seam to top out at around 2150, BNH of 12 to 12.5 will work good at 2400. Any thing 15 or over will go up to whatever I can stand to shoot. A Linotype 152gr paper patch loaded in a 300RUM shoots 3611 with good accuracy.

[Larry Gibson]

Bass

"Rifling height also determines RPM threshold if you want to call it that. The lower the rifling height you have, the lower your threshold will be before you go screwy. Get it low enough and you will have to shoot rock hard at 1600 - 1800 fps what RPMers refer to as the ideal operating range."

There is probably a trade off there. The deeper the rifling the more distorion of the bullet, thus the greater the imbalances for RPM to work on. I also believe the "ideal operating range of 1600-1800 fps" is more influenced by the 10" twist of Marlin microgrooved 30-30s than the lower rifling height.

Perhaps Marlin microgroove shooters could report the fps range of accuracy with thier 30-30s loaded with medium or slow burning powders?

Contender carbine shooters might respond also. I have Contender carbine barrels for .223 and 30-30. The .223 has a 12" twist and the 30-30 has an 11" twist (measured it several times). Both have shallow multi-groove rifling. I just haven't really worked that much with cast bullets in either. Guess I'll have to change that.

Larry Gibson

[45 2.1]

Perhaps Marlin microgroove shooters could report the fps range of accuracy with thier 30-30s loaded with medium or slow burning powders?

Same as jacketed or better to full power level. Reported in various articles and writeups besides Lyman manuals for over 50 years.

[felix]

Accuracy wise, microgrooves are preferred until they are chock full of lube and no longer work. Shooting with clean barrels is not the objective here. Rather, it is how many boolits can we shoot without cleaning the system to maintain an acceptable accuracy level for the sport intended. Nothing but a moving target, scientifically speaking. We have to have the boolit turning at the RPM calculated for any of this to make practical sense. That would mean 100 percent boolit grip throughout the shootout (as the boolit exits the barrel). There is no practical way to really tell what is going on except via inference using common sense. ... felix

[Bass Ackward]

Accuracy wise, microgrooves are preferred until they are chock full of lube and no longer work. Shooting with clean barrels is not the objective here. Rather, it is how many boolits can we shoot without cleaning the system to maintain an acceptable accuracy level for the sport intended. Nothing but a moving target, scientifically speaking. We have to have the boolit turning at the RPM calculated for any of this to make practical sense. That would mean 100 percent boolit grip throughout the shootout (as the boolit exits the barrel). There is no practical way to really tell what is going on except via inference using common sense. ... felix



Yes. Again I stated this incorrectly. When I said rifling height, what I should have said was drive area. That means that if you have 3 rifling that are .004 tall and someone else has 6 rifling .004 tall, then that has effectively doubled the drive area. Same with wider drive bands on a bullet. The disadvantage comes when you have to deal with fouling which is why I like taller rifling.

I believe, that as Felix says, fouling is the enemy. This is why cold weather kills me with soft lead on hunting loads. Harden the lead enough to handle the fouling and back comes accuracy. As fouling builds, the bullet must size down to pass. Effectively reducing not only the drive area of the bullet, but allowing enough slop for the bullet to cant in the bore which can eventually cause gas cutting and even more fouling.

The problem is that the more rifling you get in a bore, the more chances you have for creating a dimensional problem which is a limiter also.

Larry,

It would seem to me that a ratio of land height to groove diameter causes metal displacement. So if it is say 50 / 50 land to groove, then it doesn't matter how many times you chop it up, only 50% is displaced.

But I do suppose that it is easier to displace 50% metal if you had 8 little lands vs say only two or three. That way you are really altering balance and it probably takes more time to occur which distorts the base causing a lower limit too. Mr Pope actually like a pointed rifling as it cut resistance to forward motion. Or so the theory goes.

So a 2 groove Springfield will shoot equally with a four, but the four should have a higher possible ceiling. But that is again pressure related in my mind. Which is why I was interested in the gentleman that was doing the pure lead tests. He crapped way lower than ideal .... "RPM" levels cause his bullets lost it in the barrel.

Same with PB boolits vs a GC. Cause as the bullet length gradually leaves the muzzle, less and less metal is available to withstand the rifling tork. So a PB has a much lower ceiling than a GC. Unless you have tall enough rifling, wide enough drive bands, and a hard enough bullet. Which is why you saw wider base bands on PB bullets when mixes were tin lead.

[felix]

The big guns on the ships use triangular rifling about an inch deep. The real objective, however, is to make the lands push the boolit into itself, instead of shearing the excess off which installs tails on the base where the lands contacted the boolit. A worthwhile objective, time permitting, would be to change the lube and/or boolit composition to minimize these tails. ... felix

[Johnw...ski]

Wow, some interesting stuff here, thanks for taking the time for to post these responses.

How about some reports on favorite loads, the loads themselves are not important, what would be interesting would be the muzzle velocity (estimated
or actual would be fine), the twist of the barrel and caliber.

It would be interesting, to me anyway, to see what pattern develops.

John

[Larry Gibson]

Bass

"It would seem to me that a ratio of land height to groove diameter causes metal displacement. So if it is say 50 / 50 land to groove, then it doesn't matter how many times you chop it up, only 50% is displaced."

Yes you are now correct. I understood your mistated part. Thought you really meant higher rifling which causes more dostortion to the bullet. The way a cast bullet obturates and sets back from intertia during accelleration there may definately be a "trade off" point where higher (or deeper) rifling may not give better accuracy. I'm not sure what that heigth of rifling might be but 4, 5 and 6 groove barrels with .004" rifling heigth seem to shoot the best.

But I do suppose that it is easier to displace 50% metal if you had 8 little lands vs say only two or three. That way you are really altering balance and it probably takes more time to occur which distorts the base causing a lower limit too. Mr Pope actually like a pointed rifling as it cut resistance to forward motion. Or so the theory goes.

I'm not going to question Mr. Pope.

So a 2 groove Springfield will shoot equally with a four, but the four should have a higher possible ceiling. But that is again pressure related in my mind. Which is why I was interested in the gentleman that was doing the pure lead tests. He crapped way lower than ideal .... "RPM" levels cause his bullets lost it in the barrel.

Well I have to disagree that 2 groove Springfiels shoot equally with a four. I've shot enough of them side by side with jacketed and cast bullets to know that the 2 grooves will hold their own at short range but at 200+ yards the 4 groove barrel will edge them out. The simplest answer is; since it costs less to make a two groove barrel if they are equally accurate as a four groove then why aren't high power shooters using them? Why aren't factories making commercial rifles with them? Why aren't bench rest shooters using them. The answer should be obvious.

The military's acceptance of 2 groove barrels was that they shot within military acceptance standards for the M1903A3 which was pretty loose accuracy wise. They also were cheaper and quicker to make. Over the years that "shooting with in military specifications got misconcrewed into the myth that they shoot "as well as 4 groove barrels''. Just isn't so. Please, those of you who really, really want to send me those nice little groups shot at 50 and 100 yards please don't. Take a 2 groove and a 4 groove M1903 that shoot equally well at 100 yards and shoot them at both 300 yards with the same ammunition. Then we'll discuss it.

"Same with PB boolits vs a GC. Cause as the bullet length gradually leaves the muzzle, less and less metal is available to withstand the rifling tork. So a PB has a much lower ceiling than a GC. Unless you have tall enough rifling, wide enough drive bands, and a hard enough bullet. Which is why you saw wider base bands on PB bullets when mixes were tin lead"

Are you saying that the PB bullet is twisting itself as it leaves the barrel? I don't want to misconscrew something again but if that's what you are saying it is a totally new concept to me.

Also as to the gentleman pushing the PB soft lead bllets. His results were a perfect example of how RPM effects unbalanced bullets. His bullets suffered from obturation and setback to a greater degree than an alloyed (harder) GC'd bullet does. Thus his bullets were less accurate at a much less velocity. The same happens when a fast powder is used with a GC'd bullet. The RPM threshold will be lower because the bullet sustains more defects during accelleration and the RPM adversely effects the bullets filght. Remember that the RPM threshold I talk about is with medium and slow burning powders.

Larry Gibson

[Bass Ackward]

1.Yes you are now correct. I understood your mistated part. Thought you really meant higher rifling which causes more dostortion to the bullet. The way a cast bullet obturates and sets back from intertia during accelleration there may definately be a "trade off" point where higher (or deeper) rifling may not give better accuracy. I'm not sure what that heigth of rifling might be but 4, 5 and 6 groove barrels with .004" rifling heigth seem to shoot the best.[/B]

2. Well I have to disagree that 2 groove Springfiels shoot equally with a four. I've shot enough of them side by side with jacketed and cast bullets to know that the 2 grooves will hold their own at short range but at 200+ yards the 4 groove barrel will edge them out.

3. Are you saying that the PB bullet is twisting itself as it leaves the barrel? I don't want to misconscrew something again but if that's what you are saying it is a totally new concept to me.

4. Remember that the RPM threshold I talk about is with medium and slow burning powders.[/B]

Larry Gibson


Larry,

1. You only need tall enough rifling to allow for the ability to control the lead slug until it leaves the bore. The more impediment you use (twist rate ) to forward motion, the taller it will need to be for the same hardness. Look at muzzle loaders. How tall is that rifling? .015? .020? Does it hurt long range bullet flight? Took us almost 100 years to beat long range accuracy records shot with BP from the early 1800s. That's because it isn't how tall the rifling is, but the throat angle and (the speed) that you choose to engrave it. Muzzle loaders are straight line seaters.

This is why you have 1/2 degree tapers on the leade for benchrest rifles. This cuts deforming pressure on the base allowing inertia to be over come sooner. And I do have taller rifling on some rifles. It makes all the difference in the world for softer lead. Does it negatively affect accuracy on some areas? Sure. You don't want any taller rifling or faster twist rate than you need to deal with the pressure and bullet hardness you are going to run. Both will increase base pressure and lengthen the exposure time to it. This will tie into 3 and 4 later.

2. Two grooves make a bullet have more wind resistance that eventually slows the slug. We grew up with 2 and 4s. My two was actually the most accurate of all with the 311467, but I would grab a four if I had to shoot farther out.

3 & 4. I KNOW this concept is foreign to you. :grin: This is why you still believe in RPMs.

When a bullet exits high RPMs might take a poorly balanced slug to a 2" group from a 1", but if you are getting 3" groups and up, that is most likely from tipping. (hand out the car window effect) When the nose of that bullet leaves the protected environment of the bore, it wants to do it's own thing. The wider the meplat, the more force it will exert. As more and more of the bullet exits, you are reducing drive area. Same thing as reducing rifling height. Until that bullet leaves the impediment of that rifling, the rotational forces felt are exactly the same as they were anywhere in that bore because that bullets speed is still increasing. ESPECIALLY with SLOW powders. So here comes your RPM effect.

Muzzle pressure is higher with slow powders than with fast. Not only do you risk stripping a PB at some velocity, the air has been reacting on that nose since it emerged from the bore. The more aerodynamic the nose, the less this turbulence (force) will be. That bullet begins to try to tip which can bend the bullet. Especially if your rifling height couldn't hold it centered in the bore.

What that does is eventually force the base NOT to break square. Gas rushes out and cuts away PB balance unless you have a GC, a wad, or solid filler to protect it. But if it strips, you lost control and all hope is lost from that point on anyway. Protect the base and you will get some semblance of accuracy as the bullet goes back to sleep. You actually harden a bullet for two reasons. Pressure at the beginning and rotation at the exit. And remember, your barrel is moving (vibrating) at this time too.

But the rushing gas also acts like rocket propulsion and kicks the base the opposite direction to really throw it off coarse. This is where the really bad accuracy comes from like 3" and above at 100 yards. This effect is magnified by three factors. Case capacity, powder speed, and ..... yes, you got it, bore diameter. The smaller the bore diameter with a consistent case shape and capacity the higher the muzzle pressure will be if you reduce bore diameter. In your terms, the smaller the bore diameter, the higher RPM factor. And what comes with a decrease in bore diameter you have to have a faster twist to handle bullet weight? Fast twist rates add to both beginning and exit problems. And why slower powders give you more .... "RPM effect" with cast. :grin: Also why benchresters do better with small capacity cases to cut muzzle pressure.

RPMs is there and a factor, but cast failure is a turbulence issue from a failed launch WAY MORE than RPMs. Which is why I can shoot outta balance bullets in 35 caliber and have way less effect than most people using 30s. That's why my spitzer shot better for you too. You just may have needed a few BHN harder than I do and you were there.

Without a good BC, you gotta have a perfect launch which means you have to control that slug perfectly until exit by matching hardness not only to your pressure ....... but to your twist rate AND rifling height as well. The most important point is the release, which is why I preach bedding for barrel vibration control with cast. Free float is worthless at HV.

I sure hope I stated this clearly enough as this is the key to understanding RPM as most want to call it. Want to see for yourself. Move your target in to about 15ft and look at your holes when loads start to open up. Those holes go oblong not from RPMs, but poor launching. Tells you you need to change conditions, get a stronger bullet design, taller rifling, a harder bullet, a longer barrel, or lowering twist rate. But those bullets are ruined long before they feel the RPM effects of free flight.

[Larry Gibson]

Bass

Can't really disagree with all of that except I would have put it a little differently. However, what you still don't seem to understand is that all of those things you are talking about lead to one thing; an unbalanced bullet. It is the adverse effect of RPM on an unbalanced bullet during flight that I am talking about. It is what the RPM threshold is about. The fact that nasty terrible things happen to our beloved cast bullets during accelleration and exit from the muzzle (launch" as you put it) when we fire them are a given. No disagreement from me there. The effect of the RPM threshold occurs DURING FLIGHT, that part that is called External Ballistics. We can come up with all sorts of things that happen to cast bullets in the bore and we are right but once that bullet leaves the barrel the effects of RPM on a bullet are real and measureable.

One point we still disagree on is barrel bedding. I have been bedding rifle barrels every which way for many years. I have found with cast bullets as well as jacketed bullets that if you have an accurate barrel it will shoot better free floated. I guess I'll also defer to the cast bullet bench rest shooters and ask how many shoot with free floated barrels vs bedded barrels. I also recently, at your insistance, applied bedding pressure to the barrel of my '06 when I was testing your bullets. Made no improvement in accuracy but did cause verticle stringing.

However, just to show I have an open mind and always willing to learn I am about ready to begin the test of the 3 rifles with different twists (.308s with 10, 12 and 14" twists). I now have the 3 rifles with the M43 strain guage on them and when the weather cooperates I will begin. Part of the test will be to see if accuracy is effected by barrel bedding. I will take a couple HV loads that show promiss and test them in the 10" twist rifle. I will bed the barrel with with forend pressure in increments of 2 lbs from free floated to 10 lbs pressure. I will shoot at least 2 ten shot strings (groups) at each weight of barrel pressure. During the test of these three rifles with different twists all shots will be tested with the M43 so I will be able to report velocity, pressure and BC along with the group size. The results will be posted.

Larry Gibson

[felix]

Your best odds for accuracy is to find the best free-float load first. Then modify the pressure interference by changing the pressure location and/or its force, with preference given to location rather than to force. Must test with a cold barrel each time after modifying either to see if a walk appears in the first 5 shot string. A definite walk would be indicated by a straight line pattern and will be independent of angle. A perfect indication would show up as no boolit touching its cohort in crime (at a hunnert minimim). Back during the Warehouse era a 21.75 inch barrel EXACTLY was shown to be about optimum for many different calibers and their associated gun styles using the free float system. Keep in mind that vibes are generated during the firing-pin strike as well. A change there will also affect the overall picture. ... felix

[Johnw...ski]

So what kind of RPM verses caliber are you guys having success with?

John

[Bass Ackward]

So what kind of RPM verses caliber are you guys having success with?

John


John,

Again numbers depends on two things: 1. an accuracy standard which depends on type of gun and caliber and 2. at what range.

But the highest RPM number? Using 100 yards and 1" as a standard? Came with an 152 grain LBT spitzer this winter that was water dropped using 52.5 grains of RL15 in a 30-06. That load chronos at 2810 fps. That is close enough to 200,000 RPMs to call it. Slow fired over 50 minutes, 10 shots, .877. Minus 3 fliers, two of which I called, (36X) I would have measured right at .612" center to center.

That's one group in the cold with 10# of bedding pressure. With 8#, the same load / conditions produced 1 1/2" which I still consider accurate as I have jacketed with this gun that do no better bedded with this much pressure. Won't know what it will do year round until I shoot it over 1 year. Because of lead and pressure, (and normal barrel lengths) you have to live and die pretty much with one powder compared to jacketed where you can try several at this level to maybe do better. My jacketed stuff varies with temperature and season, so I expect this to as well.

I have a summer load with this bullet at 2600 fps using 56 grains of RL19, but it is only above 80 degrees. This is using 14 BHN cause I use it on ground hogs farther out. Above 80 it will hold an inch and change. Above 90, as the lube thins, it's a 1/2" performer. At 50 degrees, it's a 6" performer. 8# tip pressure.

When I was trying for 200,000 three years ago using a factory 311440 hollow point that weighed 135 grains, I got to three successive groups from 1 1/2" to 1 3/4 at 2890. Never crossed my mind to change my bedding at that time, but I was too lazy molding those hollow points to go farther. I still consider this accurate living and dieing with one lead friendly powder option for the pressure. If people had to do that same thing with jacketed instead of just changing power speeds, they would have a different opinion of HV cast accuracy too.

[leftiye]

No anger, No offense intended, but I just gotta ask this Larry. Are there any other factors than RPM? Why can't we agree that RPM is a factor, and have you agree that muzzle issues (or any other factor), say an angled crown for instance are also a separate factor? Factors all have effects that go on during flight, some other factors also occur during flight. We all agree that factors interact. It isn't possible that they could not interact. They're all going on at once.

B.A.'s last post here opened up my eyes to what I see is a major issue that I hadn't given much thought to reviously. Really helps give perspective to choices like peak pressure, powder to use, rifling configuration, twist, lead hardness, gas checks, and more. And yes, both before and after exiting the bore RPM plays a factor.

[Johnw...ski]

John,

Again numbers depends on two things: 1. an accuracy standard which depends on type of gun and caliber and 2. at what range.

But the highest RPM number? Using 100 yards and 1" as a standard? Came with an 152 grain LBT spitzer this winter that was water dropped using 52.5 grains of RL15 in a 30-06. That load chronos at 2810 fps. That is close enough to 200,000 RPMs to call it. Slow fired over 50 minutes, 10 shots, .877. Minus 3 fliers, two of which I called, (36X) I would have measured right at .612" center to center.

That's one group in the cold with 10# of bedding pressure. With 8#, the same load / conditions produced 1 1/2" which I still consider accurate as I have jacketed with this gun that do no better bedded with this much pressure. Won't know what it will do year round until I shoot it over 1 year. Because of lead and pressure, (and normal barrel lengths) you have to live and die pretty much with one powder compared to jacketed where you can try several at this level to maybe do better. My jacketed stuff varies with temperature and season, so I expect this to as well.

I have a summer load with this bullet at 2600 fps using 56 grains of RL19, but it is only above 80 degrees. This is using 14 BHN cause I use it on ground hogs farther out. Above 80 it will hold an inch and change. Above 90, as the lube thins, it's a 1/2" performer. At 50 degrees, it's a 6" performer. 8# tip pressure.

When I was trying for 200,000 three years ago using a factory 311440 hollow point that weighed 135 grains, I got to three successive groups from 1 1/2" to 1 3/4 at 2890. Never crossed my mind to change my bedding at that time, but I was too lazy molding those hollow points to go farther. I still consider this accurate living and dieing with one lead friendly powder option for the pressure. If people had to do that same thing with jacketed instead of just changing power speeds, they would have a different opinion of HV cast accuracy too.


Please don't misunderstand, I am not looking for maximum rpm's I am looking for what works. It seems that I keep seeing that 125,000 rpm is near the upper limit.
Then I am told by a lot of shooters their velocities and twist rates, which calculate out to 45,000 to 60,000 rpm. Just wondering what works for different people. If a range of rpm show as being effective I may have found something if not, oh well.

John

[Bass Ackward]

Please don't misunderstand, I am not looking for maximum rpm's I am looking for what works. It seems that I keep seeing that 125,000 rpm is near the upper limit.
Then I am told by a lot of shooters their velocities and twist rates, which calculate out to 45,000 to 60,000 rpm. Just wondering what works for different people. If a range of rpm show as being effective I may have found something if not, oh well.

John


John,

How can I give you a number of where you are supposed to fail? Rpms is what is blamed for when you hit a level that you can not pass for what ever reason. That's the point.

I can tell you that if you want to fail at low RPM numbers, use pure lead. If you want to fail at normal cast velocity levels use ACWW with fast powders and bore ride designs. If you want to go higher, use ACWW with slow powders and better lube and Loverin designs. If you want to fail at high RPM numbers that are close to full power, use hard lead, an LBT design and slow powders.

I just did 200,000 cause I wanted the T-Shirt.

[Johnw...ski]

John,

How can I give you a number of where you are supposed to fail? Rpms is what is blamed for when you hit a level that you can not pass for what ever reason. That's the point.

I can tell you that if you want to fail at low RPM numbers, use pure lead. If you want to fail at normal cast velocity levels use ACWW with fast powders and bore ride designs. If you want to go higher, use ACWW with slow powders and better lube and Loverin designs. If you want to fail at high RPM numbers that are close to full power, use hard lead, an LBT design and slow powders.

I just did 200,000 cause I wanted the T-Shirt.

Thanks Bass Ackward,

I guess I see your point, I was trying to use RPM's as a tool but there are just too many variables.

I will keep it in mind as well as all the other good reloading, casting, and rifle building practices and leave it at that.

John

[Larry Gibson]

leftiye;311419

"No anger, No offense intended, but I just gotta ask this Larry. Are there any other factors than RPM? Why can't we agree that RPM is a factor, and have you agree that muzzle issues (or any other factor), say an angled crown for instance are also a separate factor? Factors all have effects that go on during flight, some other factors also occur during flight. We all agree that factors interact. It isn't possible that they could not interact. They're all going on at once."

Here's the deal; factors that make a rifle inaccurate like an "angled crown" make the rifle an inaccurate one period. What I am talking about with the RPM threshold is all of you who ponder why your 311291 shoots great at 1850 fps but when you push it to 2000 fps out of your 10" twist 30-06 accuracy goes south. The point is the 311291 or any regular cast bullet out of that 10" twist 30-06 WAS accurate at 1850 fps but at 2000 fps it is not. The question is then; why isn't the 311291 as accurate at 2000 fps (or higher velocity) as it is at 1850 fps. The answer is simple; The icreased accelleration may or may not increase the imbalances of the bullet through greater obturation and set back. However, any cast bullet shot at that velocity has imbalances. At a certian point ( the RPM threshold - 125-140,000 RPM) the RPM over comes rotational stability and inaccuracy (or lessoned accuracy) is the result.

The more techniques you can use to lesson the imbalances created by obturation and setback during accelleration the higher you can push the RPM threshold. We can sit around and hallucinate all we want about what happens to the cast bullet inside the barel or on exit. All that discusion only verifies the the bullets are thus imbalanced. Bass comes up with all sorts of reasons the for nasty things to happen to cast bullets inside the barrel and on exit. That's all well and fine but what happens to the bullet inside the barrel or on exit has absolutely nothing to do with how RPM adversely effects an unbalance bullet in flight. The only connection is that what happens inside the barrel and on exit create the imballance in the bullet that RPM act upon outside the barrel during flight.

This is not about; oh gee whiz can't you and Bass just agree. It is about what is happening to the bullet in flight.

"B.A.'s last post here opened up my eyes to what I see is a major issue that I hadn't given much thought to reviously. Really helps give perspective to choices like peak pressure, powder to use, rifling configuration, twist, lead hardness, gas checks, and more. And yes, both before and after exiting the bore RPM plays a factor."

The RPM threshold only applies after exit when the bullet is in flight.

Larry Gibson

[Larry Gibson]

Thanks Bass Ackward,

I guess I see your point, I was trying to use RPM's as a tool but there are just too many variables.

I will keep it in mind as well as all the other good reloading, casting, and rifle building practices and leave it at that.

John

John

That's the problem Basss has with the RPM threshold. He believes it is a "limit" and if you pay attention to a RPM "limit" then you've limited yourself and already failed.

A "threshold" is not a "limit". As I've stated (I don't know how many times) the RPM threshold applies to regular cast bullets of common design that we cast out of WWs or #2 alloy equivelents with a BHN of 14-18. These are fired in rifle cartridges using medium to slow burning powders. The RPM threshold is about 125-140,000 RPM, not a set "limit" of a specific RPM number. The reason is obvious why it is a range of RPM; the other variables. Also, as Bass notes, with fast burning powders the RPM threshold is lower.

Now obviously Bass agrees with that , even though he apparently does not see that. But look at what he says; "If you want to fail at normal cast velocity levels use ACWW with fast powders and bore ride designs. " Thus he agrees that you will fail to achieve accuracy at high velocity with such. The reason you will fail is the RPM threshold. Bass should pay attention to the effect RPM has on the bullets in flight in those cases.

Now Bass has crossed the RPM threshold with a specially designed bullet that fits inside the '06 case neck, has minimal lube grooves and has a very small nose that does not bore ride. He uses slow powders the give 100% loading density to slow down accelleration as much as possible.

I too have shot some very nice groups with cast bullets at high velocity. However they were not reproduceable on demand. If we have to adjust the load, bullet alloy, lube and bedding pressure based on the weather conditions we need to ask is that practical and is that accuracy reproduceable on demand? Are you going to be able to load them up today and shoot accurately next week regardles of the weather? But the big question is; are they reproduceable on demand.

For what its worth I shot quite a few of Bass's bullets (he cast them and sent them to me) using his techniques and following his suggestions, including barrel pressure. I did get some pretty good accuracy out of my MOA capable '06 with a 10" twist at higher velocity, 2500+ fps and RPM well above the RPM threshold. I got better accuracy with them out of a .308 Winchester with a 12" twist at the same velocity but lower RPM. The fact remains that with that bullet out of the 10" twist '06 I got better accuracy when the RPM was within he RPM threshold at 1850 fps. What that tells me is the LBT bullet Bass uses is a very good one and withstands obturation and setback from acceleration very well. But it still has its accuracy adversly effected by RPM at high velocity. That advers effect on accuracy is much smaller than with regular cast bullet designs but it is still there. Since it resists deformation during accelleration it is just effected less than a regular cast bullet.

The regular cast bullet shooter who wants as high a velocity as he can get with good accuracy out of his regular Lyman, RCBS, Saeco, Lee, etc. bore riding design cast bullet needs to pay attention to RPM.

Larry Gibson

[Johnw...ski]

John

That's the problem Basss has with the RPM threshold. He believes it is a "limit" and if you pay attention to a RPM "limit" then you've limited yourself and already failed.

A "threshold" is not a "limit". As I've stated (I don't know how many times) the RPM threshold applies to regular cast bullets of common design that we cast out of WWs or #2 alloy equivelents with a BHN of 14-18. These are fired in rifle cartridges using medium to slow burning powders. The RPM threshold is about 125-140,000 RPM, not a set "limit" of a specific RPM number. The reason is obvious why it is a range of RPM; the other variables. Also, as Bass notes, with fast burning powders the RPM threshold is lower.

Now obviously Bass agrees with that , even though he apparently does not see that. But look at what he says; "If you want to fail at normal cast velocity levels use ACWW with fast powders and bore ride designs. " Thus he agrees that you will fail to achieve accuracy at high velocity with such. The reason you will fail is the RPM threshold. Bass should pay attention to the effect RPM has on the bullets in flight in those cases.

Now Bass has crossed the RPM threshold with a specially designed bullet that fits inside the '06 case neck, has minimal lube grooves and has a very small nose that does not bore ride. He uses slow powders the give 100% loading density to slow down accelleration as much as possible.

I too have shot some very nice groups with cast bullets at high velocity. However they were not reproduceable on demand. If we have to adjust the load, bullet alloy, lube and bedding pressure based on the weather conditions we need to ask is that practical and is that accuracy reproduceable on demand? Are you going to be able to load them up today and shoot accurately next week regardles of the weather? But the big question is; are they reproduceable on demand.

For what its worth I shot quite a few of Bass's bullets (he cast them and sent them to me) using his techniques and following his suggestions, including barrel pressure. I did get some pretty good accuracy out of my MOA capable '06 with a 10" twist at higher velocity, 2500+ fps and RPM well above the RPM threshold. I got better accuracy with them out of a .308 Winchester with a 12" twist at the same velocity but lower RPM. The fact remains that with that bullet out of the 10" twist '06 I got better accuracy when the RPM was within he RPM threshold at 1850 fps. What that tells me is the LBT bullet Bass uses is a very good one and withstands obturation and setback from acceleration very well. But it still has its accuracy adversly effected by RPM at high velocity. That advers effect on accuracy is much smaller than with regular cast bullet designs but it is still there. Since it resists deformation during accelleration it is just effected less than a regular cast bullet.

The regular cast bullet shooter who wants as high a velocity as he can get with good accuracy out of his regular Lyman, RCBS, Saeco, Lee, etc. bore riding design cast bullet needs to pay attention to RPM.

Larry Gibson

Got you Larry,

I was looking for RPM ranges that would encompass all loads and conditions.

Every load, cartridge, rifle, etc. is unique with it's own RPM threshold. I wasn't looking to push the envelope, just some comfortable place in the middle that would stand up to all situstions. Bit it's not that simple.

John

[Bass Ackward]

Got you Larry,

I was looking for RPM ranges that would encompass all loads and conditions.

Every load, cartridge, rifle, etc. is unique with it's own RPM threshold. I wasn't looking to push the envelope, just some comfortable place in the middle that would stand up to all situstions. Bit it's not that simple.

John


OK. Now I see.

45 caliber 1100 - 1300 fps.

30 caliber, 1600-1800 fps.

22 caliber 1700 to 2400 fps

Shows you the pattern to extrapolate diameters inbetween except 6.5 and 7MM.

[leftiye]

Sigh,