RPM Threshold

[joeb33050]

I read references to this, searched here and in the search sites without much result. Can someone tell me about this and maybe where I can read about it?
Thanks;
joe b.

[Razor]

Yeah, someone please share your knowledge...
I been wonderin' too.

Razor

[Junior1942]

If you'll look in the "Fouling Shot" at the equipment list for the nationals you'll find their twist and chrono velocity. It's a rare one which goes over 110,000 rpm. The high speed boys use 17 and 18 twist barrels for low rpms.

2750 fps @ 1 - 18 twist = 110,000 rpms.
1528 fps @ 1 - 10 twist = 110,016 rpms.

[montana_charlie]

The only thing I know about "RPM" is a note in the FAQ section of one of the bullet maker's sites. The question was about why do .22 bullets fragment when fired. The answer was that a commercial bullet (80 grain?) fired in a military barrel rifled for the FMJ Ball (62 grain?) would fly apart. Apparently it's common knowledge among bullet companies.

As for the rest of the "RPM" subject, I am totally ignorant...
and might even prefer to stay that way.
CM

[shooter93]

Can someone post the formula to calculate rpm's when you know the velocity and twist rate? Thanks.

[Nueces]

If S = speed, in feet per second and T = rifling twist in inches, then bullet rpm is:

720 x S/T in revs per minute.

Mark

[Larry Gibson]

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]

............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]

Lot of drop in those 22.7 miles.

[Bret4207]

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.

[joeb33050]

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]

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]

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.

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, wrong powder causing improper balistics, boolits that were not fitted to the controlling area in the case neck, throat and rifling origin among several other things. 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.

[Shiloh]

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

[felix]

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]

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]

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]

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]

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]

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.