RPM Test; a tale of three twists, Chapter 2

[sundog]

Larry, yeah, kinda knew that all along. The problem is reducing all those detrimental things to near zero. We can heap environmental conditions on top of all that, too, as in temp, humidity, pressure, wind, sun angle, brightness, etc. Damage that occurs after ignition and before exit is amplified by increased rpm. The less the damage, the less the amplification of error. Changing any condition will change the threshold.

But, unless a load is faithfully repeatable across a range of conditions, what use is it?

Patiently waiting for further testing....

[Larry Gibson]

Concur...Changes we make does change the threshold but I've noticed for some years that with regular cast bullets in rifles it doesn't change that much. If accuracy strats going bad at say 128,000 RPM and we harden the bullet so accuracy stays good through 138,000 RPM we have accomplished something but are still within the threshold. I've found over the years that it is difficult to get consistant good accuracy above the threshold, i.e. above 140,000 RPM. I never had the means to test it that accurately before as I do now. We will see if my suspicians are correct.

Larry Gibson

[runfiverun]

hmmmm.
1st the lube thing i have noticed that i use a heater to make my lube flow when
sizing, it also takes a few shots to warm the bbl and then accuracy occurs.

2nd i have always tried to match my pressures to my hardness
and have tried to tell others to do the same thing , if you are not pushing hard enough
or too hard.?

3rd as far as twist rate affecting pressure? i do know it takes more energy
to spin a bullet, and the faster you have to spin it, the more eenergy it takes
which will lower your velocity,
this may have something to do with that obturation thing and also the take care
of the boolit and it will show up on the other end.

some obsrevations

i also have proven to myself that acww will not go 1700 fps with out poor accuracy
and bed leadind no matter how much lube you use, or what pressure you do it with.

[leftiye]

sundog

Quite correct; it is all of those things that cuase the defects in the balance of the bullet while inside the barrel. It is the RPM during flight outside the barrel that causes the inaccuracy. The greater the defects, the higher the RPM the greater the inaccuracy.

Larry Gibson

Larry, Correction: It is the RPM [B] during flight outside the barrel that causes SOME OF THE (or adds to the, or aggravates the) inaccuracy. IMHO. Without rotational effects there will still be inaccuracy due to other factors. Since one cannot shoot a boolit without rotating it and get accurate results, these may be inseparable, but the other factors do still exist, and they still do cause inaccuracy. TomAYto, tomAHto.

[Larry Gibson]

Leftiye, correction: there are three types of ballistics; internal, external and terminal. Were all three the same there would be only “ballistics”. They are not the same and we must realize this. What happens inside the barrel (internal ballistics) to the bullet is not what happens out side the barrel (external Ballistics) to the bullet. What happens inside the barrel during acceleration or from a poor casting is what the RPM works on out side the barrel. The two what happens are different. One is not related to the other. The centrifugal effect of the RPM will be there whether or not the bullet is unbalanced. The unbalancing happens in the barrel, the inaccuracy happens out side the barrel when those imbalances are acted upon by the RPMs centrifugal force. No imbalance then the RPM does not act on it but the centrifugal force of the RPM is still there. Not realated but simple a cause and effect.

It is apples and oranges, not ; “TomAYto, tomAHto”

Some also are arguing that the different twists mean different pressure. Well duh okay, maybe a little but how much? Not very much really. There is ample evidence that once a bullet has gone one bearing length there is no longer any increased pressure required to keep in rotating. This has to do with Newton’s law about once in motion a body tends to stay in motion. That pertains to rotating bodies also.

Additionally some seem to think that since a bullet out of a 10” twist at 2000 fps is doing 144,000 RPM that as soon as the bullet enters the barrel it is doing that amount of RPM and therefore the centrifugal force must be the same "in the barrel". They equate this with some greater amount of pressure. Such is not true. The bullet goes from 0 to 2000 fps within the barrel so the RPM is increasing as the bullet travels down the barrel. The RPM goes from no RPM, 0 to 144,000 RPM as the bullet leaves the barrel. So we see that within the first half of the bullets travel within the barrel which happens to be where the pressure is the highest the RPM is actually quite low. The RPM of a bullet in this case at mid barrel is only 72,000 rpm give or take when the pressure is decreasing. The time pressure curves of the 10” twist are almost the same as that with the 12 and 14” twist barrels. If there were a much greater pressure difference the it would have been measured. It was not.

Larry Gibson

[felix]

Well, Larry, if the boolit is still accelerating, then the RPM is accelerating, on track, hopefully! Both take force to do so via F=MA, eh? Don't forget to consider the torque on the tail of the boolit as the forefront of the bearing surface leaves the muzzle! That difference in torque is an induced imbalance, as you know. A good-based, and tough-bodied, boolit should not see that as a fault, provided the twist is within range for the INERNALS of the boolit. ... felix

[Larry Gibson]

felix

"Don't forget to consider the torque on the tail of the boolit as the forefront of the bearing surface leaves the muzzle!"

I believe that is a "Bassism" that is still to be proven. I have recovered and inspected many, many bullets from down range as have many others and they make exactly the same observation I do. None indicate any additional signs of applied pressure to one side of the groove where this additional pressure, if it exists, would have to have exerted it self.

Kindly show me one single bullet that exhibits signs of pressure on one side of the groove where this pressure is applied.

Given that the "boolit is still accelerating" when exiting the barrel then since the front of the bullet is ahead of the rear of the bullet then it is theoretically rotating faster, right? Then is the rear of the bullet is trying to catch up with the front of the bullet in RPM, correct? So what happens to the middle of the bullet?

Now if we really want to believe this "Bassism" and think that bullets twist from the torque because the back of the bullet is rotating slower than the front of the bullet and is "catching up" after the front of the bullet exits the barrel then you're way beyond me. That falls right in there with those who want to believe that bullets go faster when exiting the barrel because of the release of pressure. I do not believe either one in the least bit.

Larry Gibson

[Larry Gibson]

leftiye

One other thing to remember; if there wasn't any centrifugal force from RPM an unbalanced bullet would fly as straight as a balanced bullet. As mentioned, the ballanced bullet is not affected by RPMs centifugal force because there is nothing for the centrifugal force to act upon. However, the centrifugal force is still there. It is the centrifugal force of the RPM that acts upon the imbalancing defects of the bullet to cause yaw, pitch and wobble. It is not the imbalances that cause yaw, pitch and wobble in bullets. Yaw, pitch and wobble are caused by RPM.

Larry Gibson

[leftiye]

Yes Larry there are any number of types of ballistics, oh well. Changing the terminology is irrelevant. These thangs all contribute to accuracy or the failure thereof. Isn't it interesting how every one in this discussion can be right? Isn't it interesting how we all agree on everything and yet still find substance for argument?

The issue to me is factors that contribute to failing accuracy, I don't care if they are in the ctg case, the leade , in the barrel, at the boolit's exit from the barrel, in flight, or in your grandma's purse. RPMs are just ONE OF THE MANY factors, even during flight. If you were to say "I want to study the effects of speed of rotation of the boolit as it affects accuracy, I would have no problem with that. The answer would be, the faster it revolves, the more effect it has. EOS

Unbalanced boolit versus balanced in the absence of centrifugal force - By this are you stipulating to boolits that fly straight without being revolved? If so they all would be accurate or inaccurate depending on all of the other factors except rpm/centrifugal force, ie. if they were accurate or not as the other factors determined. Yep, there are other thangs goin' on! They would NOT be accurate simply because centrifugal forse were negated.

[Larry Gibson]

leftiye

I'm not sure it's an argument (is that arguing?) but I rather consider it a discussion. We are in complete agreement. However as Bass has pointed out correctly; we must separate the various cause of inaccuracy and test each one separately. Otherwise we do not really know what the cause of the inaccuracy is. Unless we truly understand each cause of inaccuracy seperately we won't be able to adequately deal with it in its relationship to other causes. To pick on Bass again; he has adequately dealt with several of the major causes of inaccuracies with cast bullet using the 154 gr LBT bullet. Truly understanding what can be done by dealing with and understanding each of the causes will help us all achieve better accuracy. If we understand how harder alloys can better with stand acceleration, how we can use different powders to alter the time pressure curve, how different designs of cast bullets resist deformation under acceleration better, how to better fit the bullet to the leade and sizing it correctly and numerous other things then we can tie them together to produce a more balanced bullet. That bullet when fired at high velocity and subsequent high RPM will then be more accurate.

The other and perhaps more beneficial aspect to understanding how the RPM adversely affects the cast bullets flight comes when we are going to buy or build a rifle that we want good cast bullet accuracy in. If we want hunting cast bullets at 2200 -2400 fps then we know to get a 12” twist .308 instead of a 10” twist .308. We know that if rebarreling we should get the 12” twist or more probably a 13 or 14” twist barrel for best cast bullet accuracy at higher velocity. Understanding RPM and its affects we also know what cartridges best serve using a slower burning powder at close to 100% capacity to achieve the velocity level we want. Thus we see there are many other benefits to understanding the individual aspects of all the elements of accuracy of a cast bullet.

Larry Gibson

[leftiye]

Larry, Think philosophy. Arguement is a good word! I thought that people formulated arguements in the course of discussions. "Apology" is the presentation of an arguement in favor of a position. It's like the difference between gross frosting on a boolit, and the frosting that helps fillout. There are polite arguments, and there are heated arguments, and there are out of control arguents. And, yes, the more this happens, the more I suspect that we may actually agree. (I wasn't angry, I can't think that well if I am!)

[floodgate]

Larry:

"Some also are arguing that the different twists mean different pressure. Well duh okay, maybe a little but how much? Not very much really. There is ample evidence that once a bullet has gone one bearing length there is no longer any increased pressure required to keep in rotating. This has to do with Newton’s law about once in motion a body tends to stay in motion. That pertains to rotating bodies also."

As a retired (retarded?) physic ist, I've got to support Felix' comment on the above. So long as the bullet is accelerating in the bore, the RPM's are also increasing, and the Newtonian inertial reaction HAS to require increased pressure. Back in the early BP days, there was a lot of back and forth between proponents low- and high-twist rifling (back then, pre-Greenhill, they did not think of twist in inches per turn, but in turns - or fractions of turns - per barrel-length, since, all other things being equal, longer barrels, with correspondingly increased charges, gave higher velocities). The Baker rifles of 1803 or so traded twist (as I recall, 1/2 turn in the barrel) for speed in loading and decreased fouling: accepting the well-established loss in accuracy with the slow twist, they still far out-performed the muskets of the era.

But, none of the foregoing takes anything away from your EXTERIOR BALLISTCS study, so CARRY ON! - and keep us posted.

floodgate

[waksupi]

A most interesting thread.
Considering more pressure with a faster twist. Push a bowling ball up inclines, set at 10, 45, and 60 degrees. As the incline gets greater, more effort is used to move the object the same distance. Rifleing is an incline, and the principle should be the same.

[sundog]

Actually Ric, think of it as a screw as opposed to an incline. The more you got, the more screwed you are...

[Larry Gibson]

Leftiye

Very well put, outstanding actually. Thanks.

Larry Gibson

[Larry Gibson]

floodgate

Appreciate the support, thanks.

However regards the "Newtonian inertial reaction"; some would argue once the bullet has started to spin the inertia is very little if any as the bullet now has inertia in the direction of spin. Further retardation of that inertia as the RPM increase is little if any. As I stated many have recovered bullets down range but none have found evidence of additional pressure. The time pressure curve gives a clear picture of the engraving of the bullet into the rifling. The pressure rises and then lets off a little and then picks up again. Look at the time pressure curves on the Oehler printout I posted for 44man (I believe it is on the 1st page of this thread). Every single cast bullet rifle load shows this initial pressure let off after the bullet enters the barrel. The pressure curves show a more pronounced let off in the 12 and 14” twist barrels. This shows that the pressure backs off when the bullet starts spinning in the very first part of the barrel. There is nothing in the time pressure curves to indicate a greater pressure was required in the 10” twist to keep the bullet moving. There is an indication that a greater pressure was required to initially push the bullet into the barrel on the 10” twist but after that the pressure curves are essentially the same. I am open on this issue, my point is I've found no evidence of increased effort to move the bullet down the barrel in quicker twists. To the contrary I've evidence that supports the opposite. Like I said I'm still open on this one though.

Larry Gibson

[Larry Gibson]

waksupi

I agree, in theory it should be the same but from what I and others have found perhaps it's not the same.

Larry Gibson

[Bass Ackward]

bass
straightening the wheels would be like straightening the
twist in a bbl wouldn't it?

Works that way for everything else, so that's my theory.

[Bass Ackward]

Bass

One other reminder; you've not answered the question, "As to your test with Red Dot; define "different accuracy"." with regards to your test with Red Dot. Are you now not wanting to proceed in that direction?

Quite correct; it is all of those things that cuase the defects in the balance of the bullet while inside the barrel. It is the RPM during flight outside the barrel that causes the inaccuracy. The greater the defects, the higher the RPM the greater the inaccuracy.


Larry Gibson

Larry,

Forget the Red Dot. I'll not go into it. All it was going to show was that a poorly launched projectile was a poorly launched projectile no matter what the RPMs. But I would like an answer from you.

But again I am confused. I can take a 45 caliber rifle bullet at 57,600 RPM and make it shoot 18 inch groups in the cold. I can take the same mix in a 30 caliber and spin it at 180,000 and have it shoot 5" below 50 degrees. Which bullet do you think got more damaged, unbalanced, deformed? And it still shot better at 3 times the RPM. Both guns are MOA producers with no mechanical defects.

Bear in mind that this same bullet is stabilized at 1300 fps running 46,800 RPMs. Rpms are linear. Why then isn't accuracy linear if there are no other forces involved?

[w30wcf]

Bass Akward,
Hmmmm...... Wow! I have shot plenty of .45-70 bullets but never did I have anything approaching an 18" group. Was the sectional density / bullet fit to the barrel / bullet alloy similar to the recipe in your .30 cal.? I've pushed a softer 420 gr. 16/1lead/tin bullet to 1,850 f.p.s. (66,000 rpm) and got 4" groups @ 100 yards even with a bit of flinch. For me, a .45-70 is a bit harder to control with heavy recoiling loads than the .30 cal's.

Larry,
Great work! I will be interested to see how the harder Lino bullets do in your tests since the quicker twist at higher velocities would need a harder bullet for its best work.

You have come a long way since the "Old Gentleman" taught a 15 year old youngster how to reload .30-30 cartridges with a Lee Loader and 3118 cast bullets.

w30wcf