RPM Test; a tale of three twists, Chapter 2

[Larry Gibson]

Bass

As to the Red Dot test; the following also applies to it. If you remember the RPM threshold applies to otherwise accurate cast bullet rifles loads using medium and slow burning that are pushing higher RPM because of twist rate and velocity. We "can make" bullets/loads to be inaccurate, particularly with fast powders. By causing a lot of deformation and unbalancing through rapid acceleration the bullets, at the low twist rate of a 45 cal barrel, never attain complete rotational stability before the centrifugal force of the RPM overcame it. This occurs even at low RPM as you discovered.

RPM are only linear given the same twist with increasing or decreasing velocity. However, centrifugal force also increases as the RPMs increase. I'm not sure but perhaps the increased centrifugal force is not linear as the RPM increases (anyone know? I don’t have time to research it today.). I expect it is not as accuracy is not linear even with the same twist, i. e. the greater the RPM the inaccuracy is proportionally greater.

We do know that proportional accuracy is not linear with the same twist or when compared to other twists. Not only do my tests adequately proved that but I’m sure you and many others have also observed it in your own tests. I thoroughly expect the additional comparison tests of the 3 twists will show the same nonlinear inaccuracy.

Larry Gibson

[Larry Gibson]

w30wcf

I guess I have. I hope the "Old Gentleman" (God bless him) approves. He started me off on a wonderful adventure. Thanks for the support.

Larry Gibson

[Bass Ackward]

Larry,

According to the RPM theory, If bullet balance is the same, then accuracy would improve with lower RPMs correct?

Take one HV load in any one of those twist rate barrels and cut 12" off the barrel.

Acceleration should be the same, bullet deformation should be the same, velocity will drop so RPMs will drop. Therefore, the accuracy of the load should improve with every inch you cut off the barrel?

That removes all the variables except muzzle pressures as it will increase. It answers the question in a nut shell with no way to dispute it. If accuracy gets worse, it's pressure. If accuracy improves, it's RPMs.

[Larry Gibson]

Bass

"According to the RPM theory, If bullet balance is the same, then accuracy would improve with lower RPMs correct?"

The RPM threshold is in reference to the use of cast rifle bullets using medium and slow burning powder when we try to push them to higher velocity. It is that area of RPM (generally 125,000 to 140,000 RPM) where the centrigugal force overcomes the rotational stability and accuracy worsens. The accuracy gets worse than it was in or below the RPM threshold. If you've got good loads with well cast bullets you will have best accuracy in or below the RPM threshold. If you have poor loads that are relative inaccurate they will still be more accurate in or below the RPM threshold. It is a matter of proportional accuracy compared in and to itself.

However if the bullets are balanced the same and the loads are equal, within reasonable pressures and are consistant in ignition then the accuracy will probably be equal up to and into the RPM threshold, maybe even a little beyond. Remeber that it appears the centrifugal force is not linear, it gets stronger at higher RPM. Thus at the higher RPM, above the threshold, the accuracy will get still worse as the centrifugal force is greater. Maybe not by much depending on the extent of the imbalances in the bullets equal though they may be.

"Take one HV load in any one of those twist rate barrels and cut 12" off the barrel.

Acceleration should be the same, bullet deformation should be the same, velocity will drop so RPMs will drop. Therefore, the accuracy of the load should improve with every inch you cut off the barrel?

That removes all the variables except muzzle pressures as it will increase. It answers the question in a nut shell with no way to dispute it. If accuracy gets worse, it's pressure. If accuracy improves, it's RPMs."

That is a good assumption but it contains several caveats (those be variables BTW); it is correct only until the velocity/RPM comes back down to the level where that load produces the best accuracy in or below the RPM threshold. Also accelleration is not the same as you're removing barrel length which also shortens accelleration (that's why the velocity is less). Thus the bullet may have sustained less deformation during the lessoned acceleration. That induces several variables into your test and skews the basic premiss of this line of thinking. There are in fact variables.

You are grasping at straws again with to much postulation. It is easy to theorize what the results of an experiment may be. There is only one way to know whether you are right or wrong and what the results of an experiment are; that is to actually conduct the experiment. Since it is your theory and you want to cut barrels I'll defer the conduct of this experiment to you.

My tests are already confirming the existance of the RPM threshold and the effect increased RPM has on cast bullets. I "theorize" that further testing will confirm those facts. However I do not state that theory as fact yet becuase I have not completed the tests. Perhaps you shouldn't state your "theories" as facts until you have completed tests, leaves room for less embarassment. Want to borrow my hacksaw

Larry Gibson

[joeb33050]

Larry,

According to the RPM theory, If bullet balance is the same, then accuracy would improve with lower RPMs correct?

Take one HV load in any one of those twist rate barrels and cut 12" off the barrel.

Acceleration should be the same, bullet deformation should be the same, velocity will drop so RPMs will drop. Therefore, the accuracy of the load should improve with every inch you cut off the barrel?

That removes all the variables except muzzle pressures as it will increase. It answers the question in a nut shell with no way to dispute it. If accuracy gets worse, it's pressure. If accuracy improves, it's RPMs.

If accuracy improves after you cut the barrel off, maybe the crown is better. Or maybe you cut off the accuracy-diminishing piece of the barrel. Or maybe the rifle/load likes lower velocities.
If accuracy gets worse after you cut off the barrel, maybe the crown is NFG. Or the rifle/load likes a slightly higher velocity. Or you're having a bad day.

As the exponent of the infinite-number-of-variables-so-we-can-never-learn-anything theory, how can you leave out these and the many others? Remember, variables rule!
joe b.

[Tiger]

Gibson,

You said: As mentioned, the ballanced bullet is not affected by RPMs centifugal force because there is nothing for the centrifugal force to act upon"

On yes there is, one is air and another is turbulence. Centrifugal force causes a spinning object to push against anything trying to disturb it.

Think to this, which golf ball will travel further: one hit to cause a backspin or one hit to cause a forward spin. Also if you hit the ball to give it a left or right spin, in what direction will it travel?

I think some see a bullet spinning as fast as a Dremel grinder. Take the famous 30-06 with twist 10. Let shoot 100 yards. That is 3600 inches. Means bullet only turns 360 times before it hits the target in matter fraction second. It not like crazy bicycle wheel spinning say 120,000 rpm toward target. It not get much chance, mean time, to spin.

One thing more. Some jacketed bullet actually more accurate at very highest rpms...how you explain this.

Ralf

[leftiye]

Screw threads and rifling ARE both inclined planes. JoeB that's spelled "Bad hair day"? And also there is a very strong possibility that muzzle pressure in B.A.s shortened barrel scenario will make more negative effect than the reduced rpms will improve thangs (almost worth betting on). My best guess.

[Larry Gibson]

Ralf

"You said: As mentioned, the ballanced bullet is not affected by RPMs centifugal force because there is nothing for the centrifugal force to act upon"

On yes there is, one is air and another is turbulence. Centrifugal force causes a spinning object to push against anything trying to disturb it."

Ok, you're talking about drift with regards to the spinning bullet. However if you look at drift of a bullet you'll find it is not the centrifugal force that causes it. It is the friction between the spinning surface of the bullet and the air. As to tubulancethat is a force acting upon the bullet unless you are talking about bullet turbulence, then it wouldn't be stable would it? The centrifugal force from the RPM does not exist outside the bullet.

"Think to this, which golf ball will travel further: one hit to cause a backspin or one hit to cause a forward spin. Also if you hit the ball to give it a left or right spin, in what direction will it travel?"

Again that is drift. The golf balls movement in the direction of spin has to do with the friction of the air on the ball. It has nothing to do with centrifugal force.

"I think some see a bullet spinning as fast as a Dremel grinder. Take the famous 30-06 with twist 10. Let shoot 100 yards. That is 3600 inches. Means bullet only turns 360 times before it hits the target in matter fraction second. It not like crazy bicycle wheel spinning say 120,000 rpm toward target. It not get much chance, mean time, to spin."

You include all theelements of RPM calculation but you fail to factor time in, I. e. the velocity of the bullet. If it was as you say then every bullet out of a 10" twist would have the same RPM to 100 yards. Doesn't work that way. The "M" in RPM stands for "Minute". That means we must convert the fps to to that part of a minute. What that means is the faster the bullet gets to 100 yards the faster it makes those revolutions (the R in RPM). Thus the higher RPM.

"One thing more. Some jacketed bullet actually more accurate at very highest rpms...how you explain this."

Without a specific example it is hard to answer that question. However I will say this that many may appear to be more accurate at higher velocity but could it be simply that the most accurate load for that particular bullet is at higher RPM? Or how about simply that the jacketed bullet isable o withstand the effects of acceleration a hell of a lot better than a cast bullet.

Larry Gibson

[runfiverun]

one more here

how's about a twist gain bbl?
easy on the boolit to start, good acceleration, and predictable rpm's.

no?

how about a choke bore, or a flared end, or one with a tapered bore down to
a tight spot and then tapered back up ?
let larry finish his tests?

larry.
seriously i have thought about a twist gain bbl what say y'all ?

[Larry Gibson]

Screw threads and rifling ARE both inclined planes. JoeB that's spelled "Bad hair day"? And also there is a very strong possibility that muzzle pressure in B.A.s shortened barrel scenario will make more negative effect than the reduced rpms will improve thangs (almost worth betting on). My best guess.

Leftiye

You and Bass bet on it all you want but lopping off 12" of any of the 3 rifles in this test is out of the question for two reasons; one, I'm not going to ruin a perfectly good rifle to disprove a Bassism. Second and most important is lopping of 12" makes them illegal. Like I said Bass could do it if he wants but I'm not. I've shot enough 311291s out of 18" barrel to know the Bassism is wrong. Try it yourself with a 16 or 18" barreled .308 with a 10" twist. You will find that the RPM threshold is there just the same as with the 24" barrel I'm using for this test. The difference is I can use a slower time pressure curve to achieve the same velocity in the longer barrel which means I can push further into the RPM threshold than the shorter barrel. That's one of the "variables" Bass doesn't think is there.

Larry Gibson

[Tiger]

The golf ball goes further with correct direction spin, less pressure area is created which it follows. Something like new bow and hull design on ships cut the water to create a void of water along the length of the hull so ship pass through with less friction.

I realize the time factor with bullet spin, meant it's not spinning complete rpm at those high numbers without moving forward also. The Dremel does not have move to spin fast.

Hard to be specific with jacketed bullets more accurate at higher speed because there are so many examples. Not to do with twist is too slow.

Ralf

[Larry Gibson]

Ralf

I don't claim to know that much about shooting golf balls so I'll take your word for it. However I do know a little about shooting bullets. Drift and its effects are different from the effects of centrifugal force of RPM.

Let's take a 12" twist as the math is easier; the bullet will make 300 revolutions in traveling 100 yards. If it gets there in one second it is turning 18,000 RPM (300 x 60 seconds) If it gets there in 1/10 of a second it is turning 180,000 RPM. There is a lot of difference in centrifugal force there, eh? Thus even though they both turn the same revolutions getting there the higher velocity bullet is actually spinning much faster. If the second bullet had the same flight time of 1 second as the first how many revolutions would it have made? The answer is 3,000. So given 1 seconds of flight time for both bullets the slow bullet makes 300 revolutions and the fast bullet Makes 3,000 revolutions. See the difference?

It is true that some jacketed bullets are more accurate at higher velocity in slower twists but this has to do with rotational stability of the bullet.

Larry Gibson

[Bass Ackward]

The difference is I can use a slower time pressure curve to achieve the same velocity in the longer barrel which means I can push further into the RPM threshold than the shorter barrel. That's one of the "variables" Bass doesn't think is there.[/B]

Larry Gibson

Larry,

Look. I never expected you to lop off one of those barrels. This was another one of those tests if you are interested. But you can use a take off of something you re-barreled, shoot the HV loads in it and record the data and then cut it off. The idea here is NOT to change the load to favor a short barrel. Then you rework the whole dynamics.

This is a down and dirty that should show dramatic improvement if done with a 10 twist. Quickload says that cutting 10" off a 24" barrel should drop a 2500 fps load down to 2000 fps from a 24" tube. That takes you from 180,000 RPMs back down to 140,000. Or adjust the load for 2300 fps and 1800 with the shorter tube and start over so that when you cut it off so that you end up back in .... "the zone" ... that as of yet is to be defined. That should take a 4" group and make it 1" group without any other variable as Joe says. Because the claim here is that RPMs are solely responsible for inaccuracy. So all these little other variations should only add up to maybe a 1 1/2" group at worst according to the RPM theory. I don't think it will, but we could see for sure.

According to the singular focus of accuracy theory occurring at the lowest RPMS, then that means to stay away from 30 calibers in preference to 45 calibers for cast with stronger bullets to resit deformation and can achieve stabilization at the lowest RPMs. In fact, this RPM theory ought to apply to jacketed as well. Look at all those benchresters wasting components with small calibers and over spinning when they could be shooting 45s on up for accuracy competitions. Cause if RPMs is the all powerful accuracy force, then it can't be beaten.

Big bores, with cast or jacketed would ALWAYS be significantly more accurate than small. We can call this a Larryism.

I sent you an email asking for your email address to send you some pictures that a fella sent me. I have one for a Kathleen Gibson, it that it?

[45 2.1]

Let's take a 12" twist as the math is easier; the bullet will make 300 revolutions in traveling 100 yards. If it gets there in one second it is turning 18,000 RPM (300 x 60 seconds) If it gets there in 1/10 of a second it is turning 180,000 RPM. There is a lot of difference in centrifugal force there, eh? Thus even though they both turn the same revolutions getting there the higher velocity bullet is actually spinning much faster. If the second bullet had the same flight time of 1 second as the first how many revolutions would it have made? The answer is 3,000. So given 1 seconds of flight time for both bullets the slow bullet makes 300 revolutions and the fast bullet Makes 3,000 revolutions. See the difference? Larry Gibson

Rotational velocity is imparted by the barrel twist. It has little to do with forward velocity. The boolit still turns the same amount of revolutions in the same distance regardless of the forward velocity. Rotational velocity is proportional to your centrifugal force. Your example needs amended.

[sundog]

Pressure induces velocity. Increasing pressures induces acceleration. Most standard pressure curves show a peak somewhere in the barrel and then decrease, eventually to standard pressure when the boolit exits. So, the boolit accelerates as pressure increases, and then as the peak is passed it a) maintains velocity as pressure drops, b) continues to accelerate, or c) decelerates. My way of thinking is 'c'.

Felix, what was that barrel length those guys figured out in the Houston warehouse tests? Twenty-one and a half inches?

[Larry Gibson]

45 2.1

The example does not need ammending. You are still leaving the time ellement out. Perhaps you should realise you are wrong and understand the world is not flat. Or put the time element back in as that is what the "Per Minute" means. It is not RPD (Revolutions Per Distance) it is RPM (Revolutions Per Minute).

Larry Gibson

[Larry Gibson]

Bass

You are not readng what I am posting. I have shot plenty of the exact same loads in shorter 16-20" barreled .308s and the RPM threshold remains the same. Another couple inches off he barrel is not going to change anything.

Larry Gibson

[Tiger]

Larry Gibson

Let me throw some things at you. Take your 311291 bullet . Make velocity 2500 fps. That gives us 180,000 rpm. First look too rpm...that means revolutions per minute, yes? According to Lyman time of flight to 100 yard target is .131124 seconds. Bullet is not in flight for one minute. Let us say that it took 1 full second instead of what Lyman say. Then my question to you is did bullet turn 180,000 rpm or did it turn 180,000 rpm divided by the one second which come to 3000 turns?

Let me ask different way. If you could stop bullet right after come out muzzle but not stop rotation, all this for observation purposes, would bullet be actually spinning 180,000 rpm? I think not. Another observation. Guns do not torque very much when fired but they do recoil. If you lay rifle on bench and fire with string would not you think if bullet leaves muzzle at 180,000 rpm that not only would rifle recoil back but spin some too from rotational torque?

Someone say bullet leaving muzzle like soft copper wire spun in drill. This is such crazy comparison to bullet, how do I explain. Bullet is not like spinning flywheel. If bullet is really spinning like that would you not think that this rotational force would show up in certain targets like gel test or flesh? Take a flat plate aluminum. Put it on drill stand table but no clamps. Chuck in 1/2 inch drill and set drill rpm for say 2000 rpm. Then abruptly bring drill down fast into aluminum. What happen? Aluminun gets spun very violently. Yet bullets don't show this in target medium.

I think what I am getting at is not rpm that destroys accuracy or bullet for that fact but I think pressure.

One more thing think Larry. I have seen test where noses of jacketed bullet were damaged to see what happens. You would think the bend noses would unbalance bullet badly but yet they shot say ok. I don't like that voids unbalance bullet so bad and is amplified by rpm because I don't think bullet is really spinning that fast.

You go think and let us know what you come up with to my many questions.

Ralf

[Larry Gibson]

Sundog

Actually there is not "standard" pressure at bullet exit, there is still considerable pressure pushing on the bullet. The bullet is still increasing in velocity as the pressure drops from peak pressure. It does not "maintain" (or coast along) velocity in the barrel. There are some exceptions, i.e cases with very small capacity like the .22LR or perhaps large cases with very small charges of powder. However with the cartridges, pressures and velocities we are discussing the bullet continues to increase in velocity all the way to the end of the barrel.

The correct answer is "b".

Larry Gibson

[sundog]

When referring to 'standard' pressure, what I meant was atmospheric pressure.

I still think the correct answer is 'c'. How can a boolit be accelerating with decreasing pressure? Then, too, there is friction working against it. At best it can only maintain what velocity it has.

I'm not being argumentative. What this is about is the 'damage' that occurs to the boolit as it passes through the barrel, where the damage occurs, how much, and what further damage occurs. If everything were 'good-to-go' accuracy would be a non-issue. So, we take a 'perfect' boolit, barrel, etc., and it ain't so perfect accuracy. What happened? Of course, that's what you are testing. I'm just wondering where all the damage is taking place.