probably a dumb question - certainly not my first.

We talk about the proper twist rate eg. 1/7 1/8 1/9 1/12 etc etc but we never include barrel length in that discussion and I have always wondered why.

When somebody asks what rate of twist for a particular bullet is reccomended the topic of barrel length never comes up - maybe it doesn't matter in the equation...

Will a 16" barrel with a 1/9 twist have a better chance of stabilizing a 77gr. bullet than a 11.5" barrel with a 1/9 twist?  Could a longer barrel potentially reduce the need for a faster twist rate due to increased velocity, faster rate of spin due to the longer barrel etc?

thoughts?

I think there have been some people that have shot the 75/77gr loads out of 20" rifles that had better stabilization than in their 16" rifles.  That might just be a variance in the actual twist rates, but I really don't know.  It might have to do with the velocity and time under the influence of the rifling being longer.

Quoted: It might have to do with the velocity and time under the influence of the rifling being longer.

Exactly - that's my thinking right there.  But my idea is only based on common sense and nothing more.  I would think the longer a bullet stays in the barrel the more stable it's attitude will be as it travels down range.  So - I'm buying a 14.5" Barrel that's 1/9 - will it handle a 75gr slug or would a 16" or 18" be better.  I honestly don't know...

Increased velocity means faster rpm of the bullet means better stablization. Yes a longer barrel stablizes better than shorter barrels.

Quoted: ...So - I'm buying a 14.5" Barrel that's 1/9 - will it handle a 75gr slug or would a 16" or 18" be better.  I honestly don't know...

Buy a 1/7 and don't worry about it.

Quoted: So - I'm buying a 14.5" Barrel that's 1/9 - will it handle a 75gr slug or would a 16" or 18" be better.  I honestly don't know...

1:7 for a heavy bullet like that.

Length won't make that much difference--once it's rotating, it's stable, and there's also a factor of rotation relative to velocity.  It might actually be MORE stable with the same spin in a shorter barrel.  quite a few bullets lose oscillation (Gain stability) as they fly (of course, any deviation has already happened) and then lose it as they go transonic/subsonic.

"Why?"

IMO, Too many variables.    Harmonic weight of the barrel in question, bullet weight, bullet shape, uniformity of bullet shape (within a given manufacurer), powder choice, burn rate.


I'm sure a comparison could be made using only the mil-spec 5.56 ammo but when you widen the choices to commercially available ammo and then the reload possibilities the range of variables becomes huge.

MY 14.5 IS A 1/7 it works fine

In the case of the AR15/M16 you should be asking how the length of the barrel and location of the gas port effect velocity and what velocity is needed to stabalize a given weight projectile for a given twist rate.