Twist, rpm, accuracy

[joeb33050]

This is where I am with the chapter for the book. I've asked Pat I. for a page or so on this since I can't find what he wrote previously.
The book is a summary of what we know, so if you have a correction or criticism or opinion; I want to hear it. However, my "theory bin" is overflowing. Hang on to them.
joe b.
TWIST, RPM, ACCURACY

Do bullets shoot more accurately with slower twists = lower RPMs; assuming stability of the bullets? Doesn't mean that the other variables don't count; they do.
We have some evidence that slower twists work better = better accuracy than faster twists at higher velocities and lower RPMs.
First is the 30BR light bullet slow twist for SCORE in modern benchrest. There's a lot on the 30BR and short bullets and slow twists at http://www.6mmbr.com/30BR.html
Second is the CBA experimenters shooting higher velocities with slow twists, accurately. See The Fouling Shot, Sept.-Oct. 2008, pg. 195-18.
Third is the results of Larry's Gibson's experiments, the results of which suggest that faster twist barrels run out of accuracy earlier than slower twist barrels, as velocity and RPM increase.
Fourth is from the Berger Bullets web site at http//www.bergerbullets.com

Q: What does the recommended twist mean?
A: We list a twist rate that will produce the best stability in all situations. Factors like temperature, elevation and muzzle velocity will affect how the bullet will stabilize. The twist rates we recommend will give you the best stability at the slowest twist possible. The recommended twist is based on a range of velocity so it is a guideline not an absolute reuirement.

Q: Why do you recommend the slowest twist?
A: Spinning a bullet faster than necessary can amplify any inconsistency in the bullet. Since we use J4 jackets you can shoot Berger Bullets in faster twist than what is listed. We list the slowest twist rate needed because we want to squeeze every bit o0f accuracy out of a rifle.

My interpretation of this set of 4 lumps of evidence is that there is a STRONG suggestion that stable bullets shoot more accurately with slower twist barrels at higher velocities = lower RPM.
Not proved, proving is hard and expensive. RPM is not the sole, and probably not the most important accuracy determinator.
All the other variables, ctg., powder, primer, bullet, alloy, lube, ........ affect accuracy. To some degree.
To make the suggestion STRONGER, we need more data.
But, my interpretation of the data/evidence we do have leads me to the conclusion that I will specify the SLOWEST twist that will stabilize the planned bullet on my next re-barreling of a gun. Because, at up 200 yards, I know of no evidence that a slow twist/stable bullet combination is LESS accurate than faster twists.
If Greenhill says that the minimum twist for stability is 1:17", I know of nothing suggesting that 1:<17" is MORE accurate. For a given bullet.
That's how I see it.

I've said in another thread it's been my experience and a lot of other really good shooters I know that in a thirty caliber starting with a 10 twist at around 1850 fps you can gain about 100 fps more velocity and still maintain BEST accuracy for every inch reduction in twist. Smaller than 30 cal and you can run faster RPM, larger requires slower. Which is why you see 7 and 9 twists in the 22 calibers and 18 and 20 in the 45s
Pat Iffland.





From the Berger Bullets web site:




I suspect that short light bullets are easier to shoot accurately, hence the references to 100-125 grain .30 caliber bullets.

Greenhill says that in 30 caliber, a 10" twist will stabilize a bullet 1.42" long, which would weigh ~250 grains. It says that a 15" twist, for examploe, will stabilize a bullet .95" long weighing ~166 grains. All Cast bullets.
I think it's clear that 30 caliber twists are generally "faster than needed", that slower twists with shorter lighter bullets are easier to shoot and torque on the bags less.
My impression is that gain twist and slow twist barrels are "easier on the bullet" as the bullet accelerates in rotation.

joe b.

[runfiverun]

it looks correct to me.
the term" best accuracy " breaks it down real quick.

[Larry Gibson]

Joe

Suggest you read; Ballistics, Theory oand Design of Guns and Ammunition by Carlucci and Jacobson. Google it and you should get some sample pages to read. Don't get bogged down with the math equations but pay attention to what they say. Also look at Dan Lilja's article on "A Look AT Bullet Imbalance and TWIST"." I think if you do a little more research you will find there is more than a "Strong suggestion" that slower twists giving less RPM give better accuracy. There is a lot of scientific information out there that will or should lead anyone to more than a strong conclusion regarding barrel twists and the effects of RPM on bullets, jacketed and cast.

Larry Gibson

[Wayne Smith]

I don't know if you have addressed this elswhere in the book, but people should be reminded that the original Greenhill formula was developed for artillery shells and may need some modification to use with boolits.

I assume you are well aware of this, just reminding you. I know how easily details can sometimes get lost.

[Larry Gibson]

I agree with Wayne. While the Greenhill Formula and the most usual other one used (forget the name - CRAFT) most often work quite well for jacketed bullets. However they fall short with cast bullets. The other one is the formula that gives a factor of 1.3 as stable and 1.5 and maximum stability. That formula seems to work ok if the cast bullet is longer than 1". However shorter cast bullets like 311291 or 31141 all give results that seem to be in left field.

Larry Gibson

[44man]

There is a lot more to it then what is stated. It is true that if you shoot only to 200 yd's, over spinning a bullet will open groups because of the high RPM's making the bullet rotate around the flight path. Many of these conditions work themselves out at 300 yd's and farther and long range groups can be much smaller then close range groups. Close range target shooters lean towards slower twists.
If all shooting is done at extreme range, I would not go with a slow twist. I see it all the time with my BPCR where I have shot sub 1" groups at 200 meters but the entire berm and the hills behind it are safe at 500 meters. Creating this underspun condition makes it very hard to find a boolit that will be stable at long range and a very tiny change in the boolit has a profound effect. This can be seen clearly with the Marlin .44 and the 1 in 38" twist. A boolit can be made to shoot way under an inch at 50 yd's yet will not stay on paper at 100. Once the velocity bleeds off, the boolit goes unstable like the gyroscope that starts to wobble all over the table.
Another thing never mentioned is barrel length. What effect does a certain twist rate have with a revolver barrel of, say, 7" compared to the same twist in a 24" barrel? Now how about a 3" barrel, if the velocity is the same with all three examples. Will a 3" barrel impart the same RPM's on a boolit that the 24" barrel does?
I don't know the answer to that! But in the real world, the velocities are very different for each length and spin will be very different. A heavy boolit that shoots great at the rifle velocity can not be spun up enough in the short barrel because of the velocity limit.
I would still prefer a little faster spin because it is easier to slow down the bullet for close range and you can speed it up for long range. The faster twist rates are not as picky with different bullets either. With a slow twist, you might only find ONE bullet.
Caliber also has a large effect. I have a Swede that will shoot every weight bullet from 100 gr's to 140 gr's to the same POI and less then 1/2" groups at 100 yd's. and all of you know about the super fast spin rate they have.
Everything so far stated in the posts are correct if you sit at a bench punching tiny holes in paper but let's face it, most of us here do not shoot BR and many are hunters. If all the different boolits/bullets used by all of us are listed, it would take a week to read them all.
Even with revolvers, twist is very important and the absolute, most accurate are the BFR's. I have shot 500 meter groups with them that are as good as a lot of other revolvers get at 25 yd's. The twist rates are FASTER then all the other revolvers. The BFR 45-70 has a twist of 1 in 14". It has kept 5 shots in 7/16" at 50 yd's, shoots under an inch at 100 and shoots better then most rifles at 500 meters.
Would I trade for a slower twist? Not on your life!
All of this slow twist talk is moot unless the use of the gun, the distances shot, the bullets intended to be used and a thousand other things are also figured in. I would never tell anyone to buy a slower twist then normal because I know he will be disappointed if he is a normal shooter with 4 or 5 boolit molds.
Greenhill can not be adjusted to fit everything. You might do it for ONE boolit in ONE barrel at ONE velocity. Even then it will give too slow of a twist for usefulness. So what do you do? Try to make it fit all the other combinations? Seems to me you can find what your gun shoots best and then make Greenhill agree, backwards if you ask me.

[felix]

44man, freebore is the main killer with boolits. Revolters have plenty by definition. You need to check the difference of land impression on a fired boolit with the land width in the barrel. If the difference is very, very close to zero on the BASE of the boolit, then that boolit got the full rotation as prescribed by the twist. You might find that this difference is not zero, especially up towards the nose, which implies slippage, and that would be expected with such a big diameter boolit. The accuracy you describe indicates your boolits do indeed get enough rotation, slippage or no slippage. Something to check out. In other words, the twist formulas assume 100 percent land grabbing at least at the boolit tail. ... felix

[Bass Ackward]

Joe,

You have nailed this perfectly. I wouldn't change a word. Publish it for all posterity.

About time that those CBA benchrest boys learned that they all should be shooting 38 twist 44s or 45 calibers instead of playing with those RPMy toy guns that just aren't capable of being accurate.

[leftiye]

Starts to sound like slow twist is a one boolit special scenario sometimes. Like my pustulating on another thread about minimum lube capacity grooves and a super lube. You canonly make it work with that good of a lube (though it may well work superbly)..

[Pat I.]

All of this slow twist talk is moot unless the use of the gun, the distances shot, the bullets intended to be used and a thousand other things are also figured in. I would never tell anyone to buy a slower twist then normal because I know he will be disappointed if he is a normal shooter with 4 or 5 boolit molds.

I'd agree with a lot of what you're saying if 99% of the people were shooting 500 yds and beyond but since it's more likely the exact opposite where the 99% are shooting 200 yds and below I don't think too many would be disappointed with a slower twist.

I'll agree with you that sometimes a group farther out might have a smaller MOA than a group shot closer but thinks it's impossible for the actual group size to be smaller unless affected by outside influences.

I know it's often stated that most of you here don't shoot benchrest but if you're shooting a rifle off a bench it's benchrest and you're doing it to try to achieve the best accuracy you can get. If you think about it that way how many don't shoot benchrest??

[44man]

Felix, you are entirely correct. ALL of my best shooting revolver boolits show a wider land mark at the top of the boolit but are at the proper dimension at the boolit base. I understand this and have ignored it when accuracy is at it's best. It really means little and can't be fixed without going to condom bullets anyway. The only other option is a gain twist.
My 45-70 BFR has a HUGE space to move before the forcing cone and rifling is reached. But it has zero effect on accuracy.
It takes a God awful slow boolit to get the same size marks from the lands.
Lead is soft no matter what the alloy. There is no way to engage and hold rifling marks evenly with ANY high velocity. And trying to estimate the RPM's a boolit goes to pot might actually be the point a boolit skids a long way down the bore and the gas seal is destroyed and NOT that it is spun too fast.
The 6.5 Swede also has a very long freebore and it does not effect accuracy but beware using too slow a powder because SEE can happen.
All of these tests being done are ignoring other factors that come into play to destroy results and the blame is placed on other then what might be the real problem. If a super fast boolit strips the entire length of the barrel, how can anyone say it was spun too fast????? There is NO WAY to isolate only one condition.

[Larry Gibson]

44man

You have it a little backwards. Not debating the accuracy results you observe but there is probably another reason. Slower twists are selected for accuracy at longer range as long as the bullet is stable at the muzzle. Velocity decays a lot faster than RPM. Thus as range increases there is a higher RPM to velocity ratio and the bullets become more stable. A bullet that is at maximum stabilization or over stablized at the muzzle is really over spun as range increases. Accuracy may very well result from that with both cast and jacketed bullets.

Larry Gibson

[joeb33050]

44man

You have it a little backwards. Not debating the accuracy results you observe but there is probably another reason. Slower twists are selected for accuracy at longer range as long as the bullet is stable at the muzzle. Velocity decays a lot faster than RPM. Thus as range increases there is a higher RPM to velocity ratio and the bullets become more stable. A bullet that is at maximum stabilization or over stablized at the muzzle is really over spun as range increases. Accuracy may very well result from that with both cast and jacketed bullets.

Larry Gibson

The BPCR guys disagree, and with a lot of shooting and data. It seems that a given bullet/twist combination is stable at shorter ranges and becomes unstable at longer ranges. This with big bores and slow velocities. This is addressed in the book, with alternatives to Greenhill. The RPM:Velocity ratio doesn't seem to control.

BTW, look at Lilja/Twist, see another stating that slower twist = better accuracy-his article derives from Col. Harrison's work.

While we don't have much DATA, the opinions of respected folk are clustering about the minimum twist notion.

joe b.

[joeb33050]

Joe,

You have nailed this perfectly. I wouldn't change a word. Publish it for all posterity.

About time that those CBA benchrest boys learned that they all should be shooting 38 twist 44s or 45 calibers instead of playing with those RPMy toy guns that just aren't capable of being accurate.

I can't decide if you didn't read it, can't understand it, don't agree with it, are being difficult, or just don't express yourself well. I have always had a tough time understanding what you write. It ain't me.
Respectfully Yours;
joe b.

[joeb33050]

44man, freebore is the main killer with boolits. Revolters have plenty by definition. You need to check the difference of land impression on a fired boolit with the land width in the barrel. If the difference is very, very close to zero on the BASE of the boolit, then that boolit got the full rotation as prescribed by the twist. You might find that this difference is not zero, especially up towards the nose, which implies slippage, and that would be expected with such a big diameter boolit. The accuracy you describe indicates your boolits do indeed get enough rotation, slippage or no slippage. Something to check out. In other words, the twist formulas assume 100 percent land grabbing at least at the boolit tail. ... felix

I used to believe that freebore business until I got the Competitor pistol, which is chambered such that the base of any cast bullet I have, dropped in the chamber, is .15"-.4" in FRONT of the case mouth. Any bullet seated GC in mouth of the 30BR case comes nowhere near touching the rifling. But the gun shoots. (10" twist GM barrel)
Baffles me.
joe b.

[joeb33050]

I don't know if you have addressed this elswhere in the book, but people should be reminded that the original Greenhill formula was developed for artillery shells and may need some modification to use with boolits.

I assume you are well aware of this, just reminding you. I know how easily details can sometimes get lost.

I love and admire Greenhill, have never know forecast to fail at reasonable V and shorter (vice BPCR) ranges. Twists of many rifles are published and available. Compare the twist used in the rifle, commonly used bullets, length of those bullets and Greenhill. You'll shortly become a convert.
joe b.

[44man]

44man

You have it a little backwards. Not debating the accuracy results you observe but there is probably another reason. Slower twists are selected for accuracy at longer range as long as the bullet is stable at the muzzle. Velocity decays a lot faster than RPM. Thus as range increases there is a higher RPM to velocity ratio and the bullets become more stable. A bullet that is at maximum stabilization or over stablized at the muzzle is really over spun as range increases. Accuracy may very well result from that with both cast and jacketed bullets.

Larry Gibson

Now you are getting confusing! Stable at the muzzle means nothing at range. You are correct that spin decays at a slower rate then velocity but then you agree with me that over spin can result in better accuracy at long range! Isn't that exactly what I have been saying all along? You are agreeing with me and telling me I am wrong in the same sentence.
Spin stabilization is always more important then velocity.
We are BOTH agreeing that a boolit spun too slow will lose stability at long range and that a faster spin at the muzzle can give better accuracy at long range even if the stability at the muzzle is less.
We are BOTH right but you keep saying I am wrong. It is time for us to get together!

[44man]

I'd agree with a lot of what you're saying if 99% of the people were shooting 500 yds and beyond but since it's more likely the exact opposite where the 99% are shooting 200 yds and below I don't think too many would be disappointed with a slower twist.

I'll agree with you that sometimes a group farther out might have a smaller MOA than a group shot closer but thinks it's impossible for the actual group size to be smaller unless affected by outside influences.

I know it's often stated that most of you here don't shoot benchrest but if you're shooting a rifle off a bench it's benchrest and you're doing it to try to achieve the best accuracy you can get. If you think about it that way how many don't shoot benchrest??

Pat, I spent many years shooting varmints at long range, to better then 600 yd's. My long range rifle would NOT shoot small groups at 100 yd's. But the rifle would shoot 5 shots into 1/4" at 350 yd's and I could head shoot chucks to over 600 yd's.
I suggest you take a spotting scope and watch bullets fly for yourself. Experience is the ultimate teacher, not some book crap.

[Larry Gibson]

Joe and 44man

I am not confused at all. Remember I told 44man to basically look for other causes. The other cause is "It seems that a given bullet/twist combination is stable at shorter ranges and becomes unstable at longer ranges. This with big bores and slow velocities. " The bullets become unstable at the longer ranges because the velocity is low for the weight of the bullet and twist relationship and thus the bullet is only marginally stable at the muzzle. It was determined many years ago by Frankford Arsenal (1880s) to be exact that the 22" twist was as accurate as the then in vogue 18" twist out to a very long range (past 1000 yards) with the 500 gr m881 service bullet. BPCR guys started shooting 500+ gr bullets that were only marginally stable at close range and then were even more unstable at longer range in 22" twists. They then went to 18" twists and then to faster twists as velocities got lower with heavier and heavier bullets. It's funny you mention this because that's exactly why Frankford Arsenal ran their test in the 1880s because the target shooters were using 18" twists with heavier bullets than 500 gr and Frankford Arsenal proved to their Army marksman that the 500 gr M1881 bullet was just as accurate out of the 22" twist of the TD out past any distance they were target shooting at. The 500 gr RN service bullet was at maximum stability in a 22" twist. A sharp pointed 500 gr bullet or 500+ gr bullets become marginally stable out of the 22" twist of the TD. Twists of 18" or faster are required to bring those bullets to maximum stability, especially at the slower (1100 fps and less) velocities used with those heavier bullets.

The BPCR boys are simply matching the twist of the barrel to the weight of the bullet and the muzzle velocity used to bring the bullet to maximum stability at the muzzle so the bullet maintains stability across the spectrum of flight. Not rocket science. The heavier bullets the BPCR boys becan to use were marginally stablized at the muzzle. This means accuracy may have been good at 100 yards or even 200 yards but beyond that they lost accuracy rapidly because the bullet was only marginally stabilized to begin with. Note that I stated; A bullet that is at maximum stabilization or over stablized at the muzzle is really over spun as range increases. I said "maximum or over" not "minimum". This is why 44man's faster twist revolvers are also very accurate at long range. The post I made is still correct.

BTW Joe; I advised you to read Lilja on barrel twist. He is correct. Note that "slower twist is better" as long as the bullet is at maximum stability at the muzzle, not at the minimum level of stability at the muzzle. That is what the BPCR boys understand and agree upon. I agree with them and my post agree's with them.

Larry Gibson

[Larry Gibson]

44man

[I][I spent many years shooting varmints at long range, to better then 600 yd's. My long range rifle would NOT shoot small groups at 100 yd's. But the rifle would shoot 5 shots into 1/4" at 350 yd's and I could head shoot chucks to over 600 yd's.
/I]

1/4" at 350 yards! That is .07 moa!!!!!! Your level of accuracy that you manage with everything you shoot astounds me! Why is it we do not find you as the reigning champion at all the National matches (rifle and handgun) at Camp Perry?

Larry Gibson