Twist rates and bullet weights

[Bulletski]

Hi all:
My question concerns how barrel twist rates jive with bullet weights.
Over time I’ve read so much info that seem contradictory regarding this.
So, to get to the point:
Are tighter twist rates generally better for lighter or heavier bullets. AND is the length of the bullet much of a factor?

Thanks for replies

 

[Jim Watson]

Faster twist for heavier (longer) bullet.
The length of the bullet is the MAIN factor. A 180 gr spitzer takes a faster twist than a 180 gr roundnose.

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

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JBM - Calculations - Stability

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[Txhillbilly]

Bullet length has more to do with it.

 

[Dave DeLaurant]

The current math for calculating optimum twist rates is beyond me, but nice people have created online calculators to do it for me.

While I understand that it has been rendered obsolete, the older Greenhill Formula is easier for a numpty like me to understand and produces a result that was long considered close enough:



where:

• C = 150 (use 180 for muzzle velocities higher than 2,800 ft/s)
• D = bullet's diameter in inches
• L = bullet's length in inches
• SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

https://www.vcalc.com/wiki/AndrewBudd/Greenhill+Formula+for+Optimal+Rifling+Twist+Rate

 

[mm93]

Bullet length will be fairly close to the same weight from bullet to bullet, unless one is a large flat point, or hollow base, or spire point. Heavier bullets usually like a tighter twist.
But if you look back historically at what kind of twist rates were used in the 1800's or early 1900's, and compare group sizes for top shooters back then it might make you question the faster for heavier or longer theory. Back in the 1874 World Long Rang Creedmoor matches US shooters were using a 20" twist rate with a 530 gr. bullet, and shooting extremely small groups at 800-1000 yds. But today shooters will commonly use 16" and even some with 14" twist for the same type of shooting, and same distances. How did those old shooters shoot so well with their slower twist rates, when nothing has changed in the last 150 years for this type of shooting? Magic?

 

[Dave DeLaurant]

Bullet length will be fairly close to the same weight from bullet to bullet, unless one is a large flat point, or hollow base, or spire point. Heavier bullets usually like a tighter twist.
But if you look back historically at what kind of twist rates were used in the 1800's or early 1900's, and compare group sizes for top shooters back then it might make you question the faster for heavier or longer theory. Back in the 1874 World Long Rang Creedmoor matches US shooters were using a 20" twist rate with a 530 gr. bullet, and shooting extremely small groups at 800-1000 yds. But today shooters will commonly use 16" and even some with 14" twist for the same type of shooting, and same distances. How did those old shooters shoot so well with their slower twist rates, when nothing has changed in the last 150 years for this type of shooting? Magic?

There's a better than average discussion thread on the slow muzzleloader twist rates here, including an interesting list of modern and repro twist rates:

Twist rate and bullet selection

I've read that the the accuracy of a particular gun is greatly influenced by the twist rate of the barrel and the bullet selected by the shooter. That said, I never see any discussion on how to choose the bullet. Short of just buying a bunch of bullets/sabots and shooting countless shots, what...

www.modernmuzzleloader.com

 

[Riomouse911]

OP, to your question regarding slow vs fast, the general rule is faster twists will stabilize longer and/or heavier bullets.

The original 5.56x45 twist rate of 1/12 is good for 50-55 gr .224 in spitzer bullets, it might stabilize 62 gr .224 spitzer bullets, but the longer/heavier 75 gr and up .224 spitzer bullets tumble because the twist rate is too slow.

A 1/7 twist will stabilize 55 gr, 62 gr as well as 75+ gr .224 bullets. (You may not get the best accuracy with the lighter bullets, but I don’t shoot finely enough to see much difference.)

https://bergerbullets.com/the-state-of-the-art-in-rifle-bullet-stability/#:~:text=It's%20always%20OK%20to%20spin,this%20is%20a%20rare%20problem.

There is a thing to watch for with fast-twist barrels and light varmint bullets fired from speed demons like the .22-250, .220 Swift, etc. Several gun writers have written of experiences where high speed cartridges can actually have varmint bullets come apart mid flight if spun too fast for their construction. (.223 Rem/5.56x45 is my max for .22 cal centerfire, I have fired no bullets that blew up in my 1/7 twist rifles.)

It is always a good idea to tailor your load to your rifle’s twist rate to find that happy place.

Stay safe.

 

[Jim Watson]

Back in the 1874 World Long Rang Creedmoor matches US shooters were using a 20" twist rate with a 530 gr. bullet, and shooting extremely small groups at 800-1000 yds. But today shooters will commonly use 16" and even some with 14" twist for the same type of shooting, and same distances. How did those old shooters shoot so well with their slower twist rates, when nothing has changed in the last 150 years for this type of shooting? Magic?

Bigger targets. The Creedmoor target had a 36" bull for 5 points and a 54" center for 4 points. The outer was 3 points, the "wings" 2 points.



There was one 75/75 shot.

There is a thing to watch for with fast-twist barrels and light varmint bullets fired from speed demons like the .22-250, .220 Swift, etc. Several gun writers have written of experiences where high speed cartridges can actually have varmint bullets come apart mid flight if spun too fast for their construction. (.223 Rem/5.56x45 is my max for .22 cal centerfire, I have fired no bullets that blew up in my 1/7 twist rifles.)

I tried a .223 for F-TR. It has a 28" barrel 6.5" twist, throated for 90 grain Sierras. By the time I overloaded them enough to stay supersonic to 1000 yards, I was losing occasional bullets in flight. I managed to catch one on paper at 100 that was bent. When I loaded 75 grain Hornadys hot, my spotter could see them completely disintegrating in midair. I finally got good shooting with 90 grain JLK and Berger VLDs, tough enough to stand the forces.

 

[DMW1116]

Bullet length will be fairly close to the same weight from bullet to bullet, unless one is a large flat point, or hollow base, or spire point. Heavier bullets usually like a tighter twist.
But if you look back historically at what kind of twist rates were used in the 1800's or early 1900's, and compare group sizes for top shooters back then it might make you question the faster for heavier or longer theory. Back in the 1874 World Long Rang Creedmoor matches US shooters were using a 20" twist rate with a 530 gr. bullet, and shooting extremely small groups at 800-1000 yds. But today shooters will commonly use 16" and even some with 14" twist for the same type of shooting, and same distances. How did those old shooters shoot so well with their slower twist rates, when nothing has changed in the last 150 years for this type of shooting? Magic?

Bullet diameter and velocity changed. Those old timers weren’t shooting very many 6 mm or 30 cal bullets above 2800 fps. Larger diameter spinning things are more stable than smaller diameter spinning things, given a similar spin rate. Therefore larger diameters can spin slower and get the same stability.

Also, velocities then were lower than now. Slower moving things need less spin to be stable.

 

[Varminterror]

In general, there is nothing new under the sun, so rather than relying upon formulae for calculating potential stability, it's generally well known and easily communicated which combinations of velocity and specific bullets - not just bullet weight - will stabilize.

Bullet length does matter, but bullet weight also matters.

 

[Varminterror]

if you look back historically at what kind of twist rates were used in the 1800's or early 1900's, and compare group sizes for top shooters back then it might make you question the faster for heavier or longer theory. Back in the 1874 World Long Rang Creedmoor matches US shooters were using a 20" twist rate with a 530 gr. bullet, and shooting extremely small groups at 800-1000 yds. But today shooters will commonly use 16" and even some with 14" twist for the same type of shooting, and same distances. How did those old shooters shoot so well with their slower twist rates, when nothing has changed in the last 150 years for this type of shooting? Magic?

I don't think this is really apt...

Quite bluntly, they weren't shooting small groups at 800-1000 yards, let along "extremely small" by today's standards... A perfect score on the original targets used could have been around 15x larger groups than the current 1,000yrd world record...

The game has changed a lot in the past century, the targets have changed, the gear has changed, bullets, barrels, rifles, optics have all changed... The bleeding edge of the blade in 1,000yrd competition is firing 1:7-1:10" twist with short action cartridges pushing 100-220grn bullets in 6mm to 30cal... Nothing talking about 14-16" twist except for handicap classes and matches using defunct rounds which aren't leading the competition...

 

[conan32120]

Y'all are kinda dancing around the true answer ... rpm's. a slower twist with a faster moving bullet can be just as Accurate and stable as a fast twist slower bullet

 

[mm93]

I don't think this is really apt...

Quite bluntly, they weren't shooting small groups at 800-1000 yards, let along "extremely small" by today's standards... A perfect score on the original targets used could have been around 15x larger groups than the current 1,000yrd world record...

The game has changed a lot in the past century, the targets have changed, the gear has changed, bullets, barrels, rifles, optics have all changed... The bleeding edge of the blade in 1,000yrd competition is firing 1:7-1:10" twist with short action cartridges pushing 100-220grn bullets in 6mm to 30cal... Nothing talking about 14-16" twist except for handicap classes and matches using defunct rounds which aren't leading the competition...

The game hasn't changed at all. Long range black powder shooting is still done at the same distances it's been done for 150 years, using the same size targets they used back in 1874.
What has changed is high power long range with modern smokeless high pressure loads, and bolt action rifles. So comparing the two is like comparing apples and oranges.
The shooters today shooting BPTR or BPCR are as I said, using much tighter twist barrels, and everything else....guns, powder, equipment, etc. hasn't changed, except that today's shooters all want faster twist barrels for the same cartridges and rifle styles used in 1874.
Don't confuse modern Hi Power long range with black powder long range.

 

[mm93]

Y'all are kinda dancing around the true answer ... rpm's. a slower twist with a faster moving bullet can be just as Accurate and stable as a fast twist slower bullet

Again, this is true for smokeless powders where you can adjust your charges quite a bit, as long as they're still within safe pressures for the guns. Using black powder that I was referring to, there's not much adjustment since the cases are full of powder. You can compress the loads a little, and will gain some velocity, but each brand of black powder reacts differently to compression, and groups may open up if compressed a bit too much. Even .060" compression could make a tighter group, or a worse group.

 

[Jim Watson]

using the same size targets they used back in 1874.

?
Kenny Wasserburger on Shiloh says they shoot on the modern NRA paper target with 44" black and 20" ten ring.

 

[DMW1116]

Again, this is true for smokeless powders where you can adjust your charges quite a bit, as long as they're still within safe pressures for the guns. Using black powder that I was referring to, there's not much adjustment since the cases are full of powder. You can compress the loads a little, and will gain some velocity, but each brand of black powder reacts differently to compression, and groups may open up if compressed a bit too much. Even .060" compression could make a tighter group, or a worse group.

The discussion from the OP isn’t limited to black powder.

Are the people using faster twists beating the slower older twists?

 

[Varminterror]

Long range black powder shooting is still done at the same distances it's been done for 150 years, using the same size targets they used back in 1874.

My point was that 150 years ago, THAT rule set and gear set represented the best in them-modern technology for the task, and commensurate targets well designed to measure the performance.

What changed is that, after those 150 years have now passed, that gear and the commensurately oversized targets required to catch bullets from it have been superseded by…. Well… pretty much everything which came after… including the NOW-modern technology which shoots circles inside the old black powder rifles. So now BPCR exists as a hobby farm with gear limitations, and unlike its status 150yrs ago, it is about as far from the bleeding edge of performance as we can get. Nobody talks anymore about a 1939 Ford Coupe when folks are discussing the power to weight ratios or lap times of modern NASCAR stock cars - simply because they no longer represent the leading edge, let alone the bleeding edge, of the technology.

Nothing talking about 14-16" twist except for handicap classes and matches using defunct rounds which aren't leading the competition...

 

[lysanderxiii]

It is, strictly speaking, neither the weight nor length alone that governs the required spin to stabilize a projectile, but the overturning moment of the bullet. The overturning moment is generated by the aerodynamic lift of the bullet, the location of the center of pressure (CP, where the lift acts), and the location of the center of gravity (CG).

A short fat .58 caliber bullet with a short ogive may be as long as a modern 6.5 mm but the CG will be further forward and the short ogive will have both smaller lift and a CP location that is closer to the CG, so the overturning moment of much smaller. It will, therefore, require less spin to stabilize it.

Below there are two projectiles that are approximately the same length, but the top one is going to be heavier, but will require less twist to stabilize it. The top one will generate less aerodynamic lift , and the lift is closer to the CG, so a much smaller overturning moment.

EDIT:
Modern match bullets are even worse in this regard as the nose typically has a void, moving the CG further back, while the CP remains forward.

 

[CraigC]

About the blackpowder references. It's about balance, it's not just weight or just length. It's the bullets weight and length relative to its diameter. A 500gr .50cal bullet needs a far slower twist than a .30cal 500gr bullet. Same for length, a 1" long .30cal bullet needs a much faster twist rate than a 1" long .50cal bullet. Not to mention in the 1800's they were limited by the bullets available. You can squeeze a swaged lead bullet down a bore with a 1-10" twist without it completely stripping the rifling and making a mess of the bore. A twist that fast was unnecessary anyway. The guns had the twist they needed to stabilize the bullets they were shooting.

The reason smallbore rifles need much faster twist rates has nothing to do with being "modern" but everything to do with the bullets being very long for their diameter.

 

[Jim Watson]

Another BP factor is that a twist no faster than absolutely necessary gathers less fouling.

The Irish complained at Creedmoor that their Rigby muzzleloaders were just as accurate as the Yanks' Remingtons and Sharps, but the breechloaders had the advantage of being easy to wipe between shots.

 

[Varminterror]

a slower twist with a faster moving bullet can be just as Accurate and stable as a fast twist slower bullet

It’s really, REALLY rare for this to play out in real life, however, and it’s typically wholly contextual. For example, short range benchrest guys shooting a 6BR at 100 yards might run a 1:12” or 1:14” twist with a 64grn Column bullet while a long range benchrest shooter shooting the same 6BR for 1000 yard competition might employ a 105grn Hammer bullet from a 1:7” twist… the 64 Column will be faster, but the shorter, lighter bullet is only rolling around 200k RPM to achieve sufficient stability for 100yrd BR, but the longer, slower, heavier 105 Hammer will be rolling around 275k RPM to be sufficiently stable to hold on long enough to reach 1,000. One of these shooters has the luxury of relatively ignoring aerodynamics, while the other is handcuffed to it.

Throwing the 1000yard load onto a 100yrd firing line will typically yield an expected outcome where the 1000yrd rifle falls slightly behind the optimized 100yrd rifle and load, but on the right day, he might hang with the pack - BUT doing the opposite, putting the 100yrd load on the 1,000yrd line… well… I’m not sure the 64 Column gets to the 1000yrd target without tumbling (falling transonic by 600yrds).

 

[Varminterror]

It is, strictly speaking, neither the weight nor length alone that governs the required spin to stabilize a projectile, but the overturning moment of the bullet. The overturning moment is generated by the aerodynamic lift of the bullet, the location of the center of pressure (CP, where the lift acts), and the location of the center of gravity (CG).

Contextually, this statement is internally contradictory.

Married with Children theme song plays: “You can’t have one without the OTHer…”

When we pick a more aerodynamic bullet for a given cartridge, a given caliber, we’re almost inherently choosing both a heavier AND longer bullet, which also has a longer lever arm between CoP and CoG. For MOST shooters, they just recognize the choice as picking a heavier bullet, even though they are really choosing a more aerodynamic bullet…

We don’t have an infinite number of combinations of ogive profiles, boattail angles and lengths, and bearing surface lengths or material densities available on the market. We can see SOME deviations within this relationship, for example, changing bullet class from a lead core to a mono-metal, or flat base to boattail, and a few outliers like examining Service Rifle bullets with stubby ogives and low BC’s against non-SR bullets which have room for longer ogives, OR sticking a polymer tip out front which adds a lot of length but proportionately less weight - but typically, within the context of a given application and a given aerodynamic demand, we really don’t have that control. If we seek a more aerodynamic bullet, then we inevitably apply a heavier bullet for a given cartridge, and we’re generally accepting it will also be longer, and have a longer moment arm, and demand a faster twist to stabilize.

Maybe unwittingly, most shooters will see that choice not as picking better aerodynamics, but rather as picking a heavier bullet. Folks move from 243win with a slow twist to 6 creed with a faster twist because they “can shoot heavier bullets.” Why? Because they want “more aerodynamic bullets”, not really because they want “heavier bullets.”

Also not surprisingly, when we want even better aerodynamics and we remove the bound of using the same caliber, we again choose a heavier bullet by going up in caliber, which also means an even longer bullet, BUT acknowledging, this choice often does not require any faster spin rate because the proportionality for CoP vs. CoG can be (relatively) retained - again, because we can’t choose infinitely different bullet profiles or substrates.

But in general, “more aerodynamic” is coupled to an interdependency of “heavier” and “longer.”

 

[mm93]

My point was that 150 years ago, THAT rule set and gear set represented the best in them-modern technology for the task, and commensurate targets well designed to measure the performance.

What changed is that, after those 150 years have now passed, that gear and the commensurately oversized targets required to catch bullets from it have been superseded by…. Well… pretty much everything which came after… including the NOW-modern technology which shoots circles inside the old black powder rifles. So now BPCR exists as a hobby farm with gear limitations, and unlike its status 150yrs ago, it is about as far from the bleeding edge of performance as we can get. Nobody talks anymore about a 1939 Ford Coupe when folks are discussing the power to weight ratios or lap times of modern NASCAR stock cars - simply because they no longer represent the leading edge, let alone the bleeding edge, of the technology.

I understand your point, but again it has nothing to do with what I originally posted in my reply.
I was referring to BPTR and BPCR matches, and as I previously said the targets, guns, powder, haven't changed in 150 years. What has changed is the twist rates that guys shooting those matches use today, and feel is mandatory to shoot these matches. My point was that the shooters back in 1874 shot as well or better than shooters today shooting the same matches, with the same guns, except using tighter twist rates today.
BPTR or BPCR is no less or more a hobby than modern High Power competitions. Unless you know someone who can make a living shooting these events today? And your comparison of Nascar cars running laps against modern cars isn't even relevant. There are no events for old 1939 cars to run in a Nascar type event today, but there is still a modern version of the Creedmoor matches run numerous places around the US, and done so with both original 1870's rifles and cartridges, or modern copies of those guns.
I don't really understand why you continue to change this to compare modern rifles and cartridges, when I have stated numerous times that my reply about twist rates was referring to the older matches, guns, and twist rates used then and today. I honestly don't care what modern High Power rifles or matches use, nor am I referring to them.
Are you one of those guys who just has to be right? Or just has to be the last reply?

 

[mm93]

Bigger targets. The Creedmoor target had a 36" bull for 5 points and a 54" center for 4 points. The outer was 3 points, the "wings" 2 points.

View attachment 1199386

There was one 75/75 shot.

View attachment 1199387



I tried a .223 for F-TR. It has a 28" barrel 6.5" twist, throated for 90 grain Sierras. By the time I overloaded them enough to stay supersonic to 1000 yards, I was losing occasional bullets in flight. I managed to catch one on paper at 100 that was bent. When I loaded 75 grain Hornadys hot, my spotter could see them completely disintegrating in midair. I finally got good shooting with 90 grain JLK and Berger VLDs, tough enough to stand the forces.

Bigger targets? Might be bigger than today's modern high power targets, but not bigger than today's Creedmoor style match targets. These modern versions of Creedmoor matches stress authenticity, and require targets to be the same sizes as were used in 1874 matches.

 

[mm93]

Bullet diameter and velocity changed. Those old timers weren’t shooting very many 6 mm or 30 cal bullets above 2800 fps. Larger diameter spinning things are more stable than smaller diameter spinning things, given a similar spin rate. Therefore larger diameters can spin slower and get the same stability.

Also, velocities then were lower than now. Slower moving things need less spin to be stable.

You're missing my point also. Since I wasn't asking about comparing old .45 and .44 Black powder cartridges to modern 6mm or .30 caliber cartridges. I was referring to identical cartridges used then, and now; not larger then to smaller now.