RPM Threshold barrel twist/velocity chart

[Larry Gibson]

frnkeore

Thought you were done with me......oh well, it had been peaceful in your absence. As usual you base arguments on misunderstandings. Suggest you re-read the parts in the beginnings of this test (Chapter I) where it is explained the accuracy degradation is not compared between barrels but each barrel is compared to itself. The "proportional accuracy" is then compared between barrels. In that comparison we are not looking for "best accuracy" or bench rest accuracy but are looking for that point at which accuracy begins to degrade as we increase velocity.

You are fan of picking out a few examples from CBA matches where the RPM Threshold has been pushed up a bit, just as I've continually said it could be. CBA matches are shot with accuracy in mind BTW and not to push a naked cast bullet of ternary alloy as fast as we can with accuracy (the object of these test....remember?). I suggest you go back to the CBA archives and research the match results going back several years. There you will find (I have researched them as others have) the greatest 99.99% of scores (the few winners and all the losers) are shot with cast bullets with in or well under the RPM threshold. How well does Mr. Bottiger's 7mm BR shoot that bullet at 2100 fps or 2200 fps? Probably not very well or he would be using that. His load is probably exceeding the RPM threshold right in the 2100 fps range which is exactly where many of us have pushed the RPM Threshold up to with 9 - 10" twists.

As goodsteel states; "all I got was that Larry is wrong because Larry is just wrong. No argument that RPM is the force that acts on imbalances (three separate critics admitted this to me over the phone). No evidence to the contrary. No logical reasoning. Nothing but emotionally charged accusations and random information."

Me thinks thou doest protest to much and you perhaps fall in that category. Why don't you prove me wrong by conducting your own tests and present the information/data here in a logical scientific manner instead of random selections from CBA matches of what other shooters do?

You observe; "Note that of the pictured barrels, used in Larry's tests that the 10 twist barrel is the thinest of the rifles and it looks like it might even have a barrel band."

Uh, what barrel band?

Larry Gibson

[Eutectic]

Would like to see how you've managed to "prove" that.
Larry Gibson

Here's your proof Larry. See posts #87 and #88. Then jump up to post #84 (between thread drift) and finally a poster has figured it out. Then one poster, a "Larry Gibson", agrees with him in post #85 !!!!!

Eutectic

http://castboolits.gunloads.com/show...ng-range/page5

[dtknowles]

Here's your proof Larry. See posts #87 and #88. Then jump up to post #84 (between thread drift) and finally a poster has figured it out. Then one poster, a "Larry Gibson", agrees with him in post #85 !!!!!

Eutectic

http://castboolits.gunloads.com/show...ng-range/page5

What both you and Mann have demonstrated is consistent with accepted theory. Let us assume that the RPM of the bullet decreases very little in the 75 yards traveled but that the velocity does decrease over that distance then the distance traveled for a single rotation of the bullet also decreased.

Using an example of a bullet fired from a 1/12 inch twist barrel at 2000 ft per second

MV = 2000 FPS
RPM = 120,000 RPM

If the velocity decreases to 1800 FPS at 75 yards but the RPM stays the same then the bullet only travels 10.8 inches per revolution at 75 yards.

120000 rpm divided by 60 is 2000 revs per second. The reciprocal of 2000 rps is .0005 seconds per rev. at 1800 fps that is .9 feet per rev or 10.8 inches per rev.

It is normal and expected that the inches per revolution will decrease the farther the bullet get from the muzzle.

Since the RPM remains the same the stability of the bullet remains the same, the distance travelled per revolution is not the driving factor in bullet stability just the RPM achieved.

Tim

[Larry Gibson]

Thanks Tim, I get what Eutectic was saying now;

"Since the RPM remains the same the stability of the bullet remains the same, the distance travelled per revolution is not the driving factor in bullet stability just the RPM achieved."

The rate of twist part of the equation is established by the twist with the bullet in the barrel in the barrel, not any distance the bullet travels outside the barrel. The measurement of the velocity (while I measure the velocity outside the barrel the M43 Computer, knowing the BC, distance to the start screen and other variables converts the measured velocity back to muzzle velocity. The velocity simply establishes the time element in the equation.

What Eutectic is talking about is the reason why the group dispersion is non linear once the bullet has gone beyond that RPM Threshold. It is most noticeable with the helical arc that increases in a larger diameter as the range increases. It is because the RPM remains constant while the distance traveled is less per the RPM. Essentially as the range increases more RPMs are traveled for the distance traveled which means the helical arc increases in a non linear fashion. Hard to get one's head wrapped around that but once there it makes perfect sense.

Larry Gibson

[Larry Gibson]

Here's your proof Larry. See posts #87 and #88. Then jump up to post #84 (between thread drift) and finally a poster has figured it out. Then one poster, a "Larry Gibson", agrees with him in post #85 !!!!!

Eutectic

http://castboolits.gunloads.com/show...ng-range/page5

Yup, slept on it over night and it clicked what you were talking about. Then I read dtknowles post and it all came together. Mea culpa on the misunderstanding. Turns out we are definitely on the same page!

Larry Gibson

[dtknowles]

So to maybe expound on Eutectic and Larry's stuff. So if a bullet is not perfectly balanced then every rotation it alters its trajectory (it is really a continuous process but breaking it down into per rotation chunks helps me visualize it) away from ideal. So in traveling to the 100 yard target the bullet rotates 32 times and produces a 1 inch group. If it only rotated 32 more times to get to the 200 yard target we would expect a 2 inch group but it actually it rotates 35 times to get from 100 to 200 yards, I would think that this only opens the group up to 2.2 inches. This was for the 2000 FPS and 120,000 rpm. If we examine 2600 FPS and 156000 rpm I get 32 rev's to 100 and 37 move revs to 200 for a 2.4 or so inch group. I used a BC of .3 which may be a little high so the effect may be more pronounced than this example shows. While it is nonlinear it is not as dramatic of an effect as Larry's experiment shows so something else must be causing the RPM effect other and the compression of the helical path. This has been a very simplified analysis and it attributes all the shot dispersion to the lack of balance of the bullet and nothing to barrel harmonics or velocity or bullet weight variations. If those were accounted for the effect would be reduced.

Tim

[357maximum]

Yup, slept on it over night and it clicked what you were talking about. Then I read dtknowles post and it all came together. Mea culpa on the misunderstanding. Turns out we are definitely on the same page!

Larry Gibson

Well gee darn Larry...NOW if I could just get you to call it the RPM "hurdle" where excellent accuracy with higher rpm gets tougher (not impossible) to achieve.......instead of threshold we too would agree almost entirely.

[garym1a2]

This " helical arc" seems aweful close in speed to the sound barrier, any corralation?

Thanks Tim, I get what Eutectic was saying now;

"Since the RPM remains the same the stability of the bullet remains the same, the distance travelled per revolution is not the driving factor in bullet stability just the RPM achieved."

The rate of twist part of the equation is established by the twist with the bullet in the barrel in the barrel, not any distance the bullet travels outside the barrel. The measurement of the velocity (while I measure the velocity outside the barrel the M43 Computer, knowing the BC, distance to the start screen and other variables converts the measured velocity back to muzzle velocity. The velocity simply establishes the time element in the equation.

What Eutectic is talking about is the reason why the group dispersion is non linear once the bullet has gone beyond that RPM Threshold. It is most noticeable with the helical arc that increases in a larger diameter as the range increases. It is because the RPM remains constant while the distance traveled is less per the RPM. Essentially as the range increases more RPMs are traveled for the distance traveled which means the helical arc increases in a non linear fashion. Hard to get one's head wrapped around that but once there it makes perfect sense.

Larry Gibson

[Larry Gibson]

dtknowles

;2832477]So to maybe expound on Eutectic and Larry's stuff. So if a bullet is not perfectly balanced then every rotation it alters its trajectory (it is really a continuous process but breaking it down into per rotation chunks helps me visualize it) away from ideal. So in traveling to the 100 yard target the bullet rotates 32 times and produces a 1 inch group. If it only rotated 32 more times to get to the 200 yard target we would expect a 2 inch group but it actually it rotates 35 times to get from 100 to 200 yards, I would think that this only opens the group up to 2.2 inches. This was for the 2000 FPS and 120,000 rpm. If we examine 2600 FPS and 156000 rpm I get 32 rev's to 100 and 37 move revs to 200 for a 2.4 or so inch group. I used a BC of .3 which may be a little high so the effect may be more pronounced than this example shows. While it is nonlinear it is not as dramatic of an effect as Larry's experiment shows so something else must be causing the RPM effect other and the compression of the helical path. This has been a very simplified analysis and it attributes all the shot dispersion to the lack of balance of the bullet and nothing to barrel harmonics or velocity or bullet weight variations. If those were accounted for the effect would be reduced.

The above would hold true if every bullet had exactly the same imbalance and exactly the same velocity. We know neither is the case. Additionally your computation is based on the linear expansion of the group + the linear expansion of the deviation. We know that is not correct. No I do not know the formula to figure it out but I'm sure there is one. What we do know is the results on target. The farther from the muzzle the greater effect the centrifugal force has. The helical arc such a bullet takes or the tangent it may go off on (the bullets above the RPM threshold can do that also) does not increase in linear fashion as range increases. Your computations compute what should be happening at 100 and 200 yards but do not take into account the continual centrifugal force on the bullet in between the muzzle and the 200 yard target. The effect is compounded.

Several mention "harmonics" or "barrel harmonics". We all know the barrels vibrate (the "harmonics") as the bullet goes down the bore. We all know that can be the cause of shot dispersion, good or bad. We all know that the shot dispersion should be linear as the range increases. However, can you explain to me and everyone else how the barrel harmonics can cause the bullets flight path to curve in a non linear fashion?

Reducing velocity variation and bullet weight variations are 2 of the ways we produce accurate loads and push the RPM Threshold up. Reducing velocity variation is how we uniform the "barrel harmonics" shot to shot. If you've read any of the additional posts on how I push the 311466 to 2600+ fps with the accuracy shown below you know I get very anal about "bullet weight variations". Don't pay attention to either and the RPM Threshold goes down. Pay very close attention to both and the RPM Threshold goes up. Note the ES/SD on the velocity printout. Not bad for 10 shots at that velocity pushing 40,000 psi with a cast 311466 at 2620 fps (corrected to muzzle). Note the maintained linear expansion of the groups to 300 yards (wind out of 9 o'clock I did not correct for). Go back and review the non linear expansion of the groups posted during the test. No, that non linear expansion is not caused by barrel harmonics or bullet weight variation. Something else happened to those bullets during acceleration. The greater the acceleration the more it happened. What could have it been? Only one thing left; bullet deformation which cause non uniform imbalances to the bullets.

Larry Gibson

Attachment 108821Attachment 108822

[Larry Gibson]

Well gee darn Larry...NOW if I could just get you to call it the RPM "hurdle" where excellent accuracy with higher rpm gets tougher (not impossible) to achieve.......instead of threshold we too would agree almost entirely.

Feel free to call it what you want as we agree it is there. I'll buy.

Larry Gibson

[Larry Gibson]

This " helical arc" seems aweful close in speed to the sound barrier, any corralation?

No correlation at all. The bullets are still well above the sound barrier when this occurs. Dropping back down through the sound barrier is another matter entirely. Thanks for the good question.

Larry Gibson

[btroj]

Well gee darn Larry...NOW if I could just get you to call it the RPM "hurdle" where excellent accuracy with higher rpm gets tougher (not impossible) to achieve.......instead of threshold we too would agree almost entirely.

Well said Mike.

I'm not interested in pushing a threshold, I want to leap a hurdle.

[cainttype]

Well gee darn Larry...NOW if I could just get you to call it the RPM "hurdle" where excellent accuracy with higher rpm gets tougher (not impossible) to achieve.......instead of threshold we too would agree almost entirely.

"Just what do we mean by “RPM threshold”. The RPM threshold is that range of RPM where almost all cast bullets will lose accuracy if exceeded. For practical purposes when using a gas checked cast bullet with a BHN of 14-18 the RPM threshold is between 125,000 RPM and 140,000 RPM. But let us remember here that a “threshold is not a ‘limit’."... Larry Gibson, 03-25-2008

Seems to me you've both been agreeing for quite some time.

[btroj]

To Goodsteel.

a few things to ponder about why no solid info from those who may be skeptical.

Leaping the rpm hurdle is no small task in many cases. To quote someone else, it isn't rocket science but it does require some attention to details and some work.

Say a guy decides to try 2 bullets, 5 alloys, and 2 powders. He shoots 4 five shot groups per range session, two each with a couple loads.

That is now 20 different combinations but we need to look at working up a powder charge. Say we try 10 charges with each powder. We are now at 200 combinations, 100 range sessions, and 1000 bullets. I would call this a minimal effort, good results may require more permutations. As we shoot and learn we may alter the original plan.

those bullets need to be cast, heat treated, and aged to get consistent results. That takes time. Lots of time. Some alloys when air cooled may need 3 months of more to settle down.

What is the point of all this? That good, solid testing takes lots of time, bullets, and powder. Those 100 range sessions need to be on days with decent weather. If I can shoot 30 weeks per year with weather that makes for good results, not too windy or hot or freezing, the. I may need a few years to get the results in.

Anyone expecting any sort of solid evidence of anything needs to realize that time is a critical factor. Sound study takes time, a plan, and execution. Shooting a few loads doesn't prove a damn thing.

Who knows, in a few years we may be able to have a good discussion on how to leap the rpm hurdle. We can't do it now because the results aren't in. Heck, I don't even have the bullets cast yet.

Yep, good science is a slow process. It takes dedication, record keeping, and a plan. Anything less is just supposition based on a single result. We also need to keep the absolute bane of science out of the equation- bias. Anyone who has truly studied science knows how easy it is to let bias creep into results. This leads to inappropriate conclusions being drawn.

Now do you see why no data has been posted?

[dtknowles]

dtknowles
The above would hold true if every bullet had exactly the same imbalance and exactly the same velocity. We know neither is the case. Additionally your computation is based on the linear expansion of the group + the linear expansion of the deviation. We know that is not correct. No I do not know the formula to figure it out but I'm sure there is one. What we do know is the results on target. The farther from the muzzle the greater effect the centrifugal force has. The helical arc such a bullet takes or the tangent it may go off on (the bullets above the RPM threshold can do that also) does not increase in linear fashion as range increases. Your computations compute what should be happening at 100 and 200 yards but do not take into account the continual centrifugal force on the bullet in between the muzzle and the 200 yard target. The effect is compounded.

Several mention "harmonics" or "barrel harmonics". We all know the barrels vibrate (the "harmonics") as the bullet goes down the bore. We all know that can be the cause of shot dispersion, good or bad. We all know that the shot dispersion should be linear as the range increases. However, can you explain to me and everyone else how the barrel harmonics can cause the bullets flight path to curve in a non linear fashion?

Reducing velocity variation and bullet weight variations are 2 of the ways we produce accurate loads and push the RPM Threshold up. Reducing velocity variation is how we uniform the "barrel harmonics" shot to shot. If you've read any of the additional posts on how I push the 311466 to 2600+ fps with the accuracy shown below you know I get very anal about "bullet weight variations". Don't pay attention to either and the RPM Threshold goes down. Pay very close attention to both and the RPM Threshold goes up. Note the ES/SD on the velocity printout. Not bad for 10 shots at that velocity pushing 40,000 psi with a cast 311466 at 2620 fps (corrected to muzzle). Note the maintained linear expansion of the groups to 300 yards (wind out of 9 o'clock I did not correct for). Go back and review the non linear expansion of the groups posted during the test. No, that non linear expansion is not caused by barrel harmonics or bullet weight variation. Something else happened to those bullets during acceleration. The greater the acceleration the more it happened. What could have it been? Only one thing left; bullet deformation which cause non uniform imbalances to the bullets.

Larry Gibson

Larry I was just trying show why I think something else must be happening to explain the results you were getting. I do agree it is RPM dependent and you may be on the right track thinking (at least I was getting that impression)it is something happening to the bullets during their spin up in the barrel. It is clear that the bullet weight and velocity variations are well understood and can be limited. The barrel harmonics effect would be linear and not RPM dependent as well so that is not it either. I was looking for an effect that was RPM dependent as the mechanism for the non-linear dispersion. The compression of the helical is just a second order linear effect that when combined with the normal linear dispersion creates a slightly non-linear dispersion but not to the extreme demonstrated in your testing so something else must be happening and it could be damage to the bullets while they are in the bore.

Tim

[357maximum]

Seems to me you've both been agreeing for quite some time.

How astute of you, we have indeed basically agreed for a long long time....Larry just picked terminology that I have disagreed with from the gitgo. I have been trying to get him to call it something other than a "threshold" for years........I guess that makes two stubborn old fools is all....I know from the Pm's that I have shared with Larry that we both would agree for the most part sitting around a well liquored up campfire....he just chooses the wrong words to describe his thoughts and I totally suck at expressing mine in a way that someone smarter than I can understand.

[Larry Gibson]

Tim

Sorry if my response came off wrong. It probably got wrapped up with other responses. I was meaning to converse and discuss the issues with you. Yes there are many variables. That's why we look at things from different angles (the different graphs of data) and work diligently to isolate the variables. Your input is appreciated.

Larry Gibson

[Larry Gibson]

How astute of you, we have indeed basically agreed for a long long time....Larry just picked terminology that I have disagreed with from the gitgo. I have been trying to get him to call it something other than a "threshold" for years........I guess that makes two stubborn old fools is all....I know from the Pm's that I have shared with Larry that we both would agree for the most part sitting around a well liquored up campfire....he just chooses the wrong words to describe his thoughts and I totally suck at expressing mine in a way that someone smarter than I can understand.

I've no problem's with anyone calling the RPM Threshold a "wall", a "hurdle" or anything else. However, we all know from past posts by 2 or 3 members that they are just waiting for me to change something and then they'd come back criticizing me for it. And to them I say; no I have not changed anything about the basics of the RPM Threshold. I (we) have learned a lot more about the cause and effects of the RPM threshold as we test and study the phenomenon. The more we know the more we add to our ability to crawl over the "wall", "hurdle" over it or "push the RPM Threshold" up. Call it what you want, won't hurt my feelings.



Larry Gibson

[Larry Gibson]

btroj

"We are now at 200 combinations, 100 range sessions, and 1000 bullets."

Yikes! You really know how to make it difficult! I did no where near that many combinations or made that many range trips. Probably gone through that many bullets though. I've worked with 3 rifles, 5 bullets, 4 alloys and 5 powders. It didn't take 200 combinations and even 50 range sessions to get to where I am. I am willing to help, no sense reinventing the wheel?

Larry Gibson

[357maximum]

I feel your pain Larry..truly I do....you are not wrong on the "I told you so" bunch.

I have just seen the HURDLE cleared too many times by a few select shooters around here to call it a wall or a threshold into some other dimension. Not 3 weekends ago I watched as a fellow member blasted the hurdle with an as issued 1903 Springfield and also watched another member go smack over the hurdle with an as issued M1 Garand. The Springer load was actually outdenting military j-word tracers on mild steel at 250 yards and doing it with about MOA accuracy with the issue sights. The hurdle can be cleared sometimes by a little and sometimes by alot...all it takes is alot of work/playing/load/alloy development......I have done it too but mine was based more on others hard work/ sheer luck/pattern recognition than it was on hard diligent work on my part.