Twist Rates and Velocity?

[MontanaRifleman]

The question is how much does twist rate affect muzzle velocity? I.e. going from a 10 twist to a 9 twist in a 308 cal rifle.

Is there any differences by cal? I.e. does a faster twist rate in a 7mm slow down a bullet more than in a 308?

 

[MudRunner2005]

Barrel length and twist rate are both contributing factors for velocity. The tighter that twist is, and the longer that barrel is, the longer and faster that bullet starts spinning.... For example, a .308 with 175gr bullets with an 11.25" twist 24" barrel will produce higher velocity and greater accuracy at distance, b/c it won't start tumbling as soon as say a 11.25" 20" barrel...

Yes, all different calibers have different FPS variations depending on twist, barrel length, load data, etc... All the different calibers that use the same diameter bullet all have differences. You have to get VERY specific when it comes to your gun. Every gun is individual. Every gun will shoot different, even if it is 2 identical guns made consecutively by the same gunsmith. It's the nature of the beast.

 

[MontanaRifleman]

Barrel length and twist rate are both contributing factors for velocity. The tighter that twist is, and the longer that barrel is, the longer and faster that bullet starts spinning.... For example, a .308 with 175gr bullets with an 11.25" twist 24" barrel will produce higher velocity and greater accuracy at distance, b/c it won't start tumbling as soon as say a 11.25" 20" barrel...

Yes, all different calibers have different FPS variations depending on twist, barrel length, load data, etc... All the different calibers that use the same diameter bullet all have differences. You have to get VERY specific when it comes to your gun. Every gun is individual. Every gun will shoot different, even if it is 2 identical guns made consecutively by the same gunsmith. It's the nature of the beast.

Actually barrel length has nothing to with how fast a bullet spins other than longer barrels usually produce more velocity which spin the bullet faster.

My question is for example, how much slower will a barrel with a 9 twist push a bullet then the same type of barrel with a 10 twist pushing the same bullet? 10 fps? 30 fps? 50 fps?

 

[Joe King]

Given the difference that you can experience from one barrel to next in the same production. How would a guy ever quantify that? When you change barrels your entering a whole new set of controls, so your results are going to be different to a point.

 

[MontanaRifleman]

Yeah Joe, I figured it would be a tough question. But comparing apples to apples, same make and type barrel I'm wondering if anyone has any experience or a good idea of the difference.

 

[paphil]

I agree that it would be hard to quantify a different velocity, the faster spin will stabilize longer and heavier bullets. They go slower because they are heavier. The only reason for the faster twist is to stabilize. I have a 22-243 with a 7 twist that shoots 80g bergers quite well but if I shoot 52 g bullets, some spin apart before getting to the target.

 

[MudRunner2005]

Actually barrel length has nothing to with how fast a bullet spins other than longer barrels usually produce more velocity which spin the bullet faster.

My question is for example, how much slower will a barrel with a 9 twist push a bullet then the same type of barrel with a 10 twist pushing the same bullet? 10 fps? 30 fps? 50 fps?

You misunderstood....The bullet SPINS faster from a longer barrel BECAUSE there is more time it is touching the rifling and being acted upon by the rifling force for stabilization purposes. The rifling actually slows the bullet down slightly other than if it were a smooth-bore gun, b/c the rifling is actually biting into the bullet's jacket. So rifling has a very very miniscule affect on the velocity.

 

[MudRunner2005]

Actually barrel length has nothing to with how fast a bullet spins other than longer barrels usually produce more velocity which spin the bullet faster.

My question is for example, how much slower will a barrel with a 9 twist push a bullet then the same type of barrel with a 10 twist pushing the same bullet? 10 fps? 30 fps? 50 fps?

There is no real way to tell you exactly, unless you have full data to base off of your EXACT load and gun specs.

 

[MontanaRifleman]

You misunderstood....The bullet SPINS faster from a longer barrel BECAUSE there is more time it is touching the rifling and being acted upon by the rifling force for stabilization purposes. The rifling actually slows the bullet down slightly other than if it were a smooth-bore gun, b/c the rifling is actually biting into the bullet's jacket. So rifling has a very very miniscule affect on the velocity.

Nope. You better do some research on that. Length of barrel has zero to do with RPM's. It's the velocity.

A bullet coming out of a 22" 10 twist barrel at 3000 fps will spin just as fast as a bullet coming out of a 30" 10 twist barrel at 3000 fps.

 

[MudRunner2005]

Nope. You better do some research on that. Length of barrel has zero to do with RPM's. It's the velocity.

A bullet coming out of a 22" 10 twist barrel at 3000 fps will spin just as fast as a bullet coming out of a 30" 10 twist barrel at 3000 fps.

SPINS....NOT TRAVELS! You keep misunderstanding what I'm saying. We are saying the same thing... I never said a longer barrel had crap to do with velocity.

VELOCITY comes from the load itself. Stability comes from the rifling and barrel length, which improves accuracy down range.

 

[MontanaRifleman]

SPINS....NOT TRAVELS! You keep misunderstanding what I'm saying. We are saying the same thing... I never said a longer barrel had crap to do with velocity.

VELOCITY comes from the load itself. Stability comes from the rifling and barrel length, which improves accuracy down range.

Someone is misunderstanding someone.... Spins = RPM's

How fast a bullet spins is dependent on twist rate and muzzle velocity only, not barrel length.... which isn't the question I'm asking.

Tighter twist puts more resistance on a bullet and it will increase pressure and reduce velocity, or reduce velocity with the same pressure.

The max load muzzle velocity for bullet coming out of a 9 twist is going to be slower than the max load velocity of the same bullet coming out of a 10 twist.

 

[MudRunner2005]

I give up. We're talking about the same thing, but for some reason this is not getting through....

Done. Out. Over it. Good luck.

 

[Edd]

The 6mmBR website says............

Does Twist Rate Make a Difference in Velocity? Yes. Our tests show that, with the 80-90gr bullets, a 1:10 or 1:12 barrel will give you as much as 80fps more velocity than a 1:8 barrel, shooting the exact same loads.

 

[MontanaRifleman]

The 6mmBR website says............

Does Twist Rate Make a Difference in Velocity? Yes. Our tests show that, with the 80-90gr bullets, a 1:10 or 1:12 barrel will give you as much as 80fps more velocity than a 1:8 barrel, shooting the exact same loads.

Thanks Edd, that is exactly what I'm looking for.

Any other experience or knowledge is much appreciated.