twist rate limits

I've never seen twist faster than 1 in 7, not saying it doesn't exist. Old flintlocks firing round balls are 1 in 60 or so, I'm sure there are slower. Joe

Two of the fastest are the Swedish Mauser and the Italian Carcano, at 1-7.87" and 1-8.47 " respectively. Both 6.5 MMs. The 93 Mauser in 7mm comes in at 1-8.66" The 6.5 Arisaka is 1-9" The barrel on my home made 45-70 is 1-22", which is a bit slow for that caliber. I got for free for roofing ( shingles) a garage years ago. I have no idea who made it. The twist rate on a trapdoor is 1-22 as well, so I figured it would work. It will shoot 500 grain bullets accurately as long as I don't push them too fast.

As a general rule, the longer and heavier the bullet, and the faster you propell it...the faster the twist rate needs to be.

tark said:

As a general rule, the longer and heavier the bullet, and the faster you propell it...the faster the twist rate needs to be.

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Huh? A long heavy bullet calls for a faster twist but a high velocity will add stability in a marginal fit.

Pistol barrels operate in a different realm but there is a lot of variation in twist.
DWM thought a 10" twist was right for the Luger but Colt gets by with 16", probably inherited from the .38 Super.

The fastest rifling pitch I can recall in a production rifle is 6.5", which was the standard for the 6mm Model 1895 Lee Navy straight-pull - it had a very long round-nosed bullet of 135 grains weight, later reduced to 112 grains. The slowest pitch I remember was 114" (IIRC, but around that figure) used in the Forsyth muzzle-loading rifles of fairly large bore (12 bore, etc.) for use with roundball at reasonable hunting ranges. Forsyth felt that the slower pitch allowed him to use larger charges for higher velocities and relatively flat trajectory, and still give acceptable hunting accuracy. Of course, I have examined antique ML arms with 'straight' rifling, so anything the maker wanted was possible. I have made custom barrels with as fast as 6" pitch for special projects and individual customers, but cannot say with what results.
And the general rule is that the longer the bullet, the faster the rifling pitch needed to attain the rate of spin to stabilize it - higher velocities with the same bullet will achieve the same spin rate from somewhat slower rifling pitch.

PRD1 - mhb - MIke

Jim Watson said:

Huh? A long heavy bullet calls for a faster twist but a high velocity will add stability in a marginal fit.

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Then why does my 45-70 start losing accuracy if I push a 500 grain bullet faster than around 1300-1400 FPS? A 1-22 twist is about as slow as one can go in a 45-70, shooting 500 gr. bullets. Pushing them faster only degrades accuracy. I know this from actual range experience.

About 5 years ago or so, GMB made a 1:6 AR barrel. Which would stabilize very heavy bullets in the 90gr area very well. Believe it got dropped since it is hard to compete with the more popular 1:7 and 1:8 twists.

My 4 bore has 104" twist. That will work for bullets up to about 2200 grains if it is loaded up pretty hot.

tark said:

Then why does my 45-70 start losing accuracy if I push a 500 grain bullet faster than around 1300-1400 FPS? A 1-22 twist is about as slow as one can go in a 45-70, shooting 500 gr. bullets. Pushing them faster only degrades accuracy. I know this from actual range experience.

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Because it loses accuracy, not stability, who knows why. That is just the way your rifle is, seen in nearly all guns that "prefer" one load over another.
Modern .45-70s typically have an 18" twist, but the BPCR guns are getting things like 540 gr Money bullets.
I see that Pedersoli makes Trapdoors with period correct 22" twist but the Sharps repros get 18".

The diameter , weight, and speed of the bullet all play a part in the physics. You change one of these and there is a consequence to the others. With a smaller diameter bullet, you generally have a faster twist and faster Mv. In the same cartridge, heavier bullets definitely benefit from faster twist.
For example, all of my Ar's are either 1/7, 1/8, or 1/9. They all shoot 55 to 77gr bullets just fine, but the 1/9 will shoot the lighter bullets better and does fine down to 45gr. The 1/7 will shoot the 77gr better, but not so good with a 45gr. 22-250 is the same exact diameter bullet, but with its 1/12 twist, it will not shoot anything over 62gr or the bullets tumble, it generally uses 35gr to 55gr bullets.
The 22-250 has a much larger case volume than .223 and theoretically should be able to shoot a heavier bullet , but they crippled it with the twist rate.

Hugger-4641 said:

The 22-250 has a much larger case volume than .223 and theoretically should be able to shoot a heavier bullet , but they crippled it with the twist rate.

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Or you can look at it the other way, the military screwed up a varmint round trying to make it into an infantry rifle load.
The .22-250 has had that 14" twist for 55 grain bullets for longer than most of its present shooters have been alive.

Jim Watson said:

Or you can look at it the other way, the military screwed up a varmint round trying to make it into an infantry rifle load.
The .22-250 has had that 14" twist for 55 grain bullets for longer than most of its present shooters have been alive.

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Yep, I wasn't trying to be derogatory of the 22-250, just using it for reference. The 22-250 spits that 55gr out at about 500fps faster than a .223 can, which makes for excellent varmint performance using lighter bullets with a flatter trajectory, out to a few 100ds anyway.

In general, twist rates are actually not exact specific values but seem to have a 'range' of values. Typically, the posted values for any given bullet is the minimum value for stability. In my observation and experience, quite a bit of overspin is better than just a little underspin.
Overspin can - in extreme cases - cause bullets to 'wobble' (yaw). In more extreme cases, overspin can cause bullets to disintegrate or come apart from internal stress.

Twist rates are given for weights in specific calibers. With this thought in mind it follows that twist could be based on a ratio between length and diameter of any bullet, with the proviso of the material being the same. The usual assumption is the bullet is a lead core with a gilding metal cover (jacket). This factor and theory is difficult as manufactured bullets of the same weight will have minute differences in alloy or shape (radius of ogive) and therefore be of slightly different length or weight. :Shrug:

Fastest twist rate in my mind is for the 6.5mm arms of the early smokeless powder era (Viz. WW1). They all seem to be in the 1 rotation in 7 - 8 inches range. P. O. Ackley (if one does not recognize the name, look him up) once recorded making a 1:5 inch twist barrel for a .219 Zipper. (He was experimenting to determine how much 'twist' effected pressure.) He found very light bullets (40 grains, I think) fired at maximum velocities would disintegrate before hitting a target at fairly close range. A heavier bullet, sent a bit slower, would register on target with a hole nearing .30 caliber.
That much twist is not recommended for anything at the moment, but who knows what may present in the future?

pharmer said:

I've never seen twist faster than 1 in 7, not saying it doesn't exist. Old flintlocks firing round balls are 1 in 60 or so, I'm sure there are slower. Joe

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Back in the olden days (1978, +/-), when I was in the AF I built 3 different black powder percussion rifle kits and 2 pistol kits from Connecticut Valley Arms. The rifles' barrels were (according to CVA) rifled at 1 turn in 66 inches.

My ancient Savage .223 has a 1-14" twist and shoots 50-55 grain bullets (the orginal design weight for .223) superbly well.

I measured it by marking a cleaning rod and pushing a tight wad down the barrel until the rod turned around once. I was delighted to find out later through net research that this was 100% correct (yay!) for this rifle.

I visualize the relationship between bullet length (weight) and twist as the difference between stabilizing a frisbee and stabilizing a broomstick (without fletching).

Terry, 230RN

1-6 is fairly common in some applications, i remember someone saying they had a custom rifle with a 1-5, ill see if i can find the post later.

Dont know about the other end of the spectrum as they are generally used on guns that have not done much with yet.

Jim Watson said:

Because it loses accuracy, not stability, who knows why.

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Does that explain the keyholes?

tark said:

Does that explain the keyholes?

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Are these lead bullets? If so, is possible you're getting some slipping at the higher velocity

8.6 Blackout has 1:3 twist. Long heavy bullet moving slowly requires fast twist.

tark said:

Then why does my 45-70 start losing accuracy if I push a 500 grain bullet faster than around 1300-1400 FPS? A 1-22 twist is about as slow as one can go in a 45-70, shooting 500 gr. bullets. Pushing them faster only degrades accuracy. I know this from actual range experience.

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When your bullets go faster with that twist rate, is it possible that they are getting some lead stripped off by the lands and grooves? If so, they may be a little small for the bore after that, allowing them to "wobble".
Or does this happen with jacketed bullets as well?

tark said:

Does that explain the keyholes?

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I didn't know keyholes.
What gun, what bullet diameter, what load?

tark said:

Two of the fastest are the Swedish Mauser and the Italian Carcano, at 1-7.87" and 1-8.47 " respectively. Both 6.5 MMs. The 93 Mauser in 7mm comes in at 1-8.66" The 6.5 Arisaka is 1-9" The barrel on my home made 45-70 is 1-22", which is a bit slow for that caliber. I got for free for roofing ( shingles) a garage years ago. I have no idea who made it. The twist rate on a trapdoor is 1-22 as well, so I figured it would work. It will shoot 500 grain bullets accurately as long as I don't push them too fast.

As a general rule, the longer and heavier the bullet, and the faster you propell it...the faster the twist rate needs to be.

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It also depends on the caliber . . .

For the same twist helix angle, say 7 degrees, close to the maximum and keep from shearing off the rifling on the bullet, you get:

For .22 caliber, a 1-in-5.59 twist
For .280 caliber, a 1-in-7.11 twist
For .30 caliber, a 1-in-7.62 twist
For .45 caliber, a 1-in-11.4 twist
For a 3 inch, a 1-in 76.2 twist
For a 105mm, a 1 turn in 105 twist.

You may have noticed that all of these are all about one turn in 25.4 calibers.

reddog81 said:

8.6 Blackout has 1:3 twist. Long heavy bullet moving slowly requires fast twist.

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I just read that article and it was the lowest by far that I've ever seen or herd of... due to shear forces I believe that takes out lead which is part of the major benifit of shooting subsonic.

AJC1 said:

I just read that article and it was the lowest by far that I've ever seen or herd of... due to shear forces I believe that takes out lead which is part of the major benifit of shooting subsonic.

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I’d be interested if lead was an option. But I’m not spending the coin involved to find out it’s not feasible.

Jim Watson said:

I didn't know keyholes.
What gun, what bullet diameter, what load?

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Well, to be honest , Jim, I don't really remember the load. I haven't done any reloading in many years. I did a lot of reloading with Reloader 7 in those days so I'm pretty sure that was the powder. The bullets were cast from scrap lead so I don't know the Brinnel hardness number. The keyholes I was getting were not classic full sideways. they were oval and clearly the bullet wasn't flying completely straight. Loon Wulf and JTHunter may have explained what happened.