Maximum twist?

[Adam in WI]

The Brux thread brought back a question that's crossed my mind from time to time. My memory fails me currently, but I remember reading in Cartridges of the World about a .22 caliber centerfire that used 100 grain bullets requiring a ~1-5" twist. I think it was a Fred Barnes creation.

My question is: Am I correct in assuming there's a theoretical maximum twist rate, where the lands and grooves cease being rifling and simply become what amounts to threads?

 

[Ned Ludd]

I suspect you would reach the point of routine jacket failures before the rifling ceased to function as such.

 

[jackieschmidt]

A gain twist might be viable in such a radical twist rate.

star at 1-8 and get progressively faster toward the muzzle.

I would think that Bartlien, who uses a infinite programmable CNC system to induce the twist In their single point rifling, could pull this off. However, with that much advance, the land marks on the bullet would be pretty wide by the time the bullet left the Barrel.

 

[6ShotsOr5?]

Adam, I think I understand your question. From a purely theoretical viewpoint and ignoring real world issues such as jacket failure per @Ned Ludd, when you reach the maximum twist rate, the lands and grooves would have to start getting narrower to accommodate any higher twist rate. I think the answer could be 1 revolution / bore circumference, as ridiculously high as that sounds... I think it’s probably a bit less than that though, thinking about the angles and all.

 

[Adam in WI]

That sort of makes sense. I was picturing it as a relationship involving the bullets size but I couldn't put it into meaningful terms.

 

[338 dude]

I’m sure rpm’s would be the limiting factor if IIRC anything over 300,000 RPMs is going to start to have jacket failure and bullets exploding before they reach the target

 

[Walt Krafft]

300 Blackout pistols shooting 220 bullets like a 1 in 5 inches twist.

 

[Dave M.]

The Brux thread brought back a question that's crossed my mind from time to time. My memory fails me currently, but I remember reading in Cartridges of the World about a .22 caliber centerfire that used 100 grain bullets requiring a ~1-5" twist. I think it was a Fred Barnes creation.

My question is: Am I correct in assuming there's a theoretical maximum twist rate, where the lands and grooves cease being rifling and simply become what amounts to threads?

The difference is a thread is a single “rifling” if you would, and generally barrels have 2,3,4 or 5 rifling grooves. At a certain point, too much rifling will “overspin” bullets and either the jacket with tear right off the bullet in flight, or the spin rate will be so high it will cause the bullet yaw and tumble. Twist rate is a fine line and with most “normal” twists for caliber, you can simply slow or speed up a bullet to optimally stabilize it.
Dave m

 

[Joe Maisto]

The difference is a thread is a single “rifling” if you would, and generally barrels have 2,3,4 or 5 rifling grooves. At a certain point, too much rifling will “overspin” bullets and either the jacket with tear right off the bullet in flight, or the spin rate will be so high it will cause the bullet yaw and tumble. Twist rate is a fine line and with most “normal” twists for caliber, you can simply slow or speed up a bullet to optimally stabilize it.
Dave m

Interesting thread. In the early 60's, I sent for and received an 03 -'a3 that was a 2 groover that had no noticeable adverse effects with accuracy using military ammunition at the range , in a prone position , 10 x jacket w/sling .

 

[geo.ulrich]

300 Blackout pistols shooting 220 bullets like a 1 in 5 inches twist.

but at what velocity?

 

[kindabitey]

300 Blackout pistols shooting 220 bullets like a 1 in 5 inches twist.

That’s probably for subsonic. @dellet could answer that for sure.

 

[Herzo]

Have to wonder if you could increase the limit using gain twist technique?

 

[dellet]

That’s probably for subsonic. @dellet could answer that for sure.

It’s a trick question, and the Blackout cartridge is probably the best test bed for this type of discussion, bullet weights from 80-265 grains, lengths from around .500” to 1.875”, twists from 1/4-1/12, anything from cast lead to lathe turned solids are readily available. Velocities range from 900-3000 fps.

With the Blackout, barrel length can be the single most important factor, if we eliminate subs. If we look at the difference between the fast twists say 1/5 and 1/6, it can mean the difference in about 16” of useful barrel.

An 8” 1/5 shooting a 110 grain Varmageddon can clock in around 2150 fps spinning around 310,000 rpm.

A 24” 1/6 you can push that same bullet to 2700 fps and spin at 325,000 rpm.

Rate of fire comes into play. At a slow pace firing, either one will shoot all day without a problem. Take the 8” to a carbine course, and you’re likely to find the the inside of your suppressor hollowed out in about 40 rounds because the bullets are coming apart at the muzzle.

There is a reason you won’t find a 1/5 twist barrel over about 8-9” shooting supers.

Some of the most popular expanding subs are a disaster in a 1/5. Sig was shopping for a few specific bullets for the 1/5, it was a narrow set of specs. The Lehigh 194 opened in flight as slow as 900 fps.

It’s real easy to figure that a bullet in a 1/5 is spinning twice as fast as a 1/10, three times a 1/15. How well will a custom 110-120 grain BR bullet hold up at 350,000+ rpm?

It doesn’t matter the cartridge, when you get into fast twists, you need to crunch the numbers and match the bullet construction to the desired spin rate and type of shooting. Blowing up bullets became kind of a hobby for a couple summers.

It really changed my thinking about Bullet choice most likely to be used, as the starting point choosing a twist. Shooting a 150 grain brass solid, that might be 1/2” longer than something like a 150 Gold dot that is basically a plated lead.

So “what is maximum twist” really is a deep rabbit hole.

 

[geo.ulrich]

The point I was making is velocity and rpm go hand in hand as velocity increase so does rpm's you can only get so fast a twist with jacketed bullets before they blow up...solids are a different animal,, funny I was involved with Gov't when they started the blackout project and they stipulated a 1:10 twist for subsonics...nuff said

 

[DaveTooley]

When thinking about extreme gain twist barrels or extreme twists in general there are limitations because of cutter geometry.

 

[Dusty Stevens]

When thinking about extreme gain twist barrels or extreme twists in general there are limitations because of cutter geometry.

Internal threading tool, 60deg, at 18tpi, 2 lead threads. Thatll do it

 

[Adam in WI]

This is all purely hypothetical, setting aside machinery or bullet limitations. A mental exercise if you will. I'm thinking at some point hot gas from gunpowder will no longer be able to move a bullet forward trying to overcome the rotation that this theoretical maximum twist is trying to impart on it. Taking the nut and bolt analogy; if you have a single grove 3/4" bore threaded 14 tpi, I don't think even a match threaded projectile will be able to be expelled from the bore.

 

[geo.ulrich]

good point

 

[dellet]

This is all purely hypothetical, setting aside machinery or bullet limitations. A mental exercise if you will. I'm thinking at some point hot gas from gunpowder will no longer be able to move a bullet forward trying to overcome the rotation that this theoretical maximum twist is trying to impart on it. Taking the nut and bolt analogy; if you have a single grove 3/4" bore threaded 14 tpi, I don't think even a match threaded projectile will be able to be expelled from the bore.

Again that will depend on bullet construction. The bullet will have to be constructed out of a material hard enough to not “strip the threads”, think shear strength, before it would stop the bullet from moving.

This is a hard cast, but the idea shows. The 1700 & 1900 are reversed in the photo.


I accomplished the same thing with a turned brass bullet that had an aluminum tip using pistol powders. I could achieve a higher velocities with a slower powder. The best explanation was starting pressure and velocity. Forcing the bullet into the lands too fast and under too much pressure, deformed the bullet. That was actually in a 1/9 twist and a 78 grain bullet.