Twist rate

[Jimbob]

Little barrel twist question. 1:14” is standard for a 458 Win and 1:16” for 460 Weatherby. This makes sense as the 460 is faster so can use a slower twist for a given bullet weight. However standard for the much slower 45-70 is 1:20” or even 1:22”. Why is that? Any reason why a 460 couldn’t be built with a 1:20”?

 

[Mekaniks]

The best place to start is with the bullets that you want to shoot.

Optimum twist rate for any bullet is Bullet dia squared x 150 and then divided by the bullet length.

Dia x Dia x 150 divided by length = twist rate in inches

 

[Hogpatrol]

Little barrel twist question. 1:14” is standard for a 458 Win and 1:16” for 460 Weatherby. This makes sense as the 460 is faster so can use a slower twist for a given bullet weight. However standard for the much slower 45-70 is 1:20” or even 1:22”. Why is that? Any reason why a 460 couldn’t be built with a 1:20”?

Velocity is also a factor. A specific amount of RPMs are needed to stabilize a bullet and maintain accuracy.

 

[Ray B]

I have wondered about this at times as there are two ways the rotation of the bullet is considered. Mentioned is the rotations per second, the other is the actual path of the bullet. with rps the spin would actually become shorter as the bullet slowed, since the resistance being met by the bullet affects the forward motion of the bullet with very little affect on its angular rotation. If the bullet follows a path it would indicate that the rate of spin per distance would be constant and as the bullets forward velocity slowed, so does its spin.

It would seem that the rps would be the "right" answer, but if so, then bullets with marginal rates of twist for a particular bullet would become more stable as the velocity decreased. But this is not the case. If a long bullet is fired in a marginally twist barrel the result generally is that it will be stable for a few hundred yards, but unstable at longer range when it has slowed. So I don't know what the correct answer is, which is why I continue to wonder about it, but in the process, I follow the formula mentioned above and follow manufacturers recommendations on the optimum rate of twist for their bullets.

 

[Jimbob]

The best place to start is with the bullets that you want to shoot.

Optimum twist rate for any bullet is Bullet dia squared x 150 and then divided by the bullet length.

Dia x Dia x 150 divided by length = twist rate in inches

I’ve seen the same rule of thumb but do that for .458” and you end up around 20-22”.

As an example:
Woodleigh 500gr FMJ is 1.388”
Woodleigh 550gr FMJ is 1.521”

(.458^2 x 150) / 1.388= 22.7”
(.458^2 x 150) / 1.521= 20.7”

 

[Jimbob]

Velocity is also a factor. A specific amount of RPMs are needed to stabilize a bullet and maintain accuracy.

That’s my point, 460 Wby uses a faster twist than a 45-70 despite them using bullets of the same or at least very similar length in most cases. Which is counterintuitive because, as you correctly pointed to, increased velocity equals increased stability for a given twist I.e. what stabilises in a 1:20 45-70 will be even more stable in a 1:20 460 Wby.
So why does the Wby use a 1:16? It seems more and more likely that it’s to stabilise monolithic solids, though a 1:20 would do that too but of course not as well. A 1:20 460 would stabilise a 550gr Woodleigh FMJ very well though so if one doesn’t want to run monolithics then I still can’t see a reason not to use a 1:20. Not much a 550gr Woodleigh can’t achieve after all.

 

[colorado]

One thing you might want to consider, particularly for the big bores, is how well they stabilize the bullet once it enters the animal. There have been several chapters of various books dedicated to this topic. My 500 Jeffrey has a 1 in 10" twist, which is probably a bit extreme (you can feel the rifle twist a bit in your hands if you don't hold the forearm relatively tightly when shooting offhand), but it's accurate as hell. So I'm good with it.

 

[Jimbob]

One thing you might want to consider, particularly for the big bores, is how well they stabilize the bullet once it enters the animal. There have been several chapters of various books dedicated to this topic.

I have heard about that and it definitely has some merit. However I’ve not read a full and in-depth article about it so can’t say I am fully convinced yet. What I would love to see is the same solid at the same velocity fired into the same media but from different twists, in .458 1:14-1:20 would do it.

 

[Hogpatrol]

Just remembered, ammo type is another factor. One can buy 45-70 solid lead bullet ammo and many shoot solid lead reloads. A fast twist like the 460 or 458 could cause the bullet to go softer or at the very least leave a bore full of lead. I had an experience shooting lead shotgun slugs in a 22 twist rifled barrel. Big mistake as afterwards, I was peeling lead strips out of the rifling.

 

[colorado]

I have heard about that and it definitely has some merit. However I’ve not read a full and in-depth article about it so can’t say I am fully convinced yet. What I would love to see is the same solid at the same velocity fired into the same media but from different twists, in .458 1:14-1:20 would do it.

Agree. I would like to see the same.

 

[Ray B]

Bill Steigers (Bitterroot Bullet Co) did a lot of testing terminal effects on bullets of different spin rates/velocities. It was done with expanding bullets but the results would have merit to solids. the main conclusion was that the spin rate needed to be sufficient to keep the gyroscopic effect sufficient that the bullet would stay point forward as it met resistance and decelerated. He found the main cause of bullet failure was when the bullet was no longer kept point forward and tumbled. Once the bullet was sideways the bullet yielded to side forces that overwhelmed the bullets integrity. With a solid (monometal) the bullet would likely stay intact, but its directional stability would be affected, the result being that it could swerve off the desired route through the animal, particularly if non-uniform resistance (side of bones) is met. Bill was a strong advocate of faster than normal (he really liked 1-9" for the 375) twists but as far as I know he did very little testing with larger bores.

 

[ZG47]

I have wondered about this at times as there are two ways the rotation of the bullet is considered. Mentioned is the rotations per second, the other is the actual path of the bullet. with rps the spin would actually become shorter as the bullet slowed, since the resistance being met by the bullet affects the forward motion of the bullet with very little affect on its angular rotation. If the bullet follows a path it would indicate that the rate of spin per distance would be constant and as the bullets forward velocity slowed, so does its spin.

My information, from a gunsmith, target shooter and hunter who competes at the Seddon Range, Trentham and in various U.S, shoots, is:

that the rotational velocity, i.e. revolutions per second (rps) remains pretty much constant until the bullet loses stability. That would, I imagine, be after it has entered the transonic zone. Also, remember that extra rotational velocity equals more friction with air and greater velocity loss, i.e. deceleration. This increased deceleration is not too important for hunting with .45 cal rifles BUT the rifling in a faster twist barrel will erode more quickly than that in a slower twist barrel, all things being equal.

N.B. Greenhill’s formula for rifling twist works for lead bullets. Jacketed bullets require different calculations, even when you restrict yourself to round/flat nose flat-base bullets. This is why a good reloading programme can be a useful guide if not a ‘crystal ball’.