Grasp on reality: 1:9 turns vs 1:7 turns...

[Nathan Krynn]

Wouldn't the length of barrel have any bearing on the twist rate you should be using? I would think that a round coming from a 20" 1:9 is going to spin somewhat faster than one from a 16" 1:9 since it has that much more time to ride the twist.
I don't know a lot about this stuff, but it seems like it would be a factor.

No this sounds logical but it does not work like this at all.

[Droppin Deuces]

No this sounds logical but it does not work like this at all.

[Nathan Krynn]

Sorry it reads rougher then i meant it.

Length does give velocity due to more time for the powder to burn and or pressure to build but don't determine your desired twist with length they are two totally different things.

[Droppin Deuces]

Sorry it reads rougher then i meant it.

Length does give velocity doe to more time for the powder to burn and or pressure to build but don't determine your desired twist with length they are two totally different things.

Oh, I know. It was a reaction to my being wrong. I mean honestly, it seems like a projectile would spin faster out of a 14.5" barrel than out of a 7" and require a different twist. I'd actually like to find some data on that.

[B Strong]

I am struck by the number of 1:9 turn m4/16 barrels on the market as opposed to 1:7 turn barrels which are inherently stock on m16 rifles. Is there THAT much of a difference between the two? I ask this because it is pretty hard (in the few weeks I have been searching) to find a true 1:7 barrel.

Thank you for your help.

Depends on what ammo you're planning to use.

1/9 will run M193/SS109 without a problem, but the heavier projectiles perform better with 1/7.

I use 75 grain stuff pretty much exclusively, so my go-to AR types are either 1/7 or 1/8.

[B Strong]

Technically, it isn't about the WEIGHT, but the LENGTH of the bullet, as well as air density and velocity.

This is undoubtedly one of the questions that is asked the most. The answer depends on many things, but here are some rough guidelines for those that don't care about the details:

1-in-14" - 55gr or less
1-in-12" - 35gr up to 55-60gr
1-in-9" - 45gr-75gr, possibly up to 77 if you're lucky
1-in-8", 1-in-7" - 45gr - 80+gr

To find out for sure if your bullet will stabilize a given bullet which is on the fringe, you'll have to try it out in YOUR rifle. Some people's 1/9 can't stabilize 75gr bullets, while others can get away with 77gr bullets in theirs. A simplified method for determining the minimum rifling twist rate for a given length of bullet is given by the Greenhill formula. A Google search will show many sources for those interested in a more in-depth discussion.

http://www.ar15.com/content/page.html?id=186

Correct.

[Nathan Krynn]

Oh, I know. It was a reaction to my being wrong. I mean honestly, it seems like a projectile would spin faster out of a 14.5" barrel than out of a 7" and require a different twist. I'd actually like to find some data on that.

The spin is the same from a 14.5" to a 20". It is the speed of the bullet that changes.

There are two forces here forward movement and the spin. You are confusing them to be the same.

The 1/7 is 1 rotation in 7 inches. That will be constant no matter the barrel length.

Length just makes the bullet go faster (velocity) but the spin is the same.

Trust me shorter barrels do not need nor want different twists.

[OutlawDon]

I've shot 75 grain match ammo out of my 1:9 twist 26" Savage to 1000 yards with accuracy and no keyholing, so velocity does matter. But again, try out different ammo for your particular barrel as each one is different.

[Nathan Krynn]

I'm not saying velocity doesn't matter I am just advising do not change the twist for barrel length.

[Droppin Deuces]

I get the difference between velocity and spin, but I just don't see how a bullet coming out of a barrel with the same twist at two different lengths can possibly spin at the same speed unless the bullet was already spinning before it contacted the rifling. It's going to take time after entering the rifling at 0rpm to build momentum to attain its ideal rotational speed. Given the same twist rate, a bullet coming from a 7" barrel can't possibly spin as fast as one coming from a 20" barrel.

That's just how it seems to me.

[PatriotnMore]

1:7 is good for longer bullets, which tend to also be heavier bullets, mil needs em for tracer rounds too. some lower weight rds don't like the tighter twists
1:9 is fine for a do everything twist. I've always heard they last longer too

fixed it.

[Nathan Krynn]

I get the difference between velocity and spin, but I just don't see how a bullet coming out of a barrel with the same twist at two different lengths can possibly spin at the same speed unless the bullet was already spinning before it contacted the rifling. It's going to take time after entering the rifling at 0rpm to build momentum to attain its ideal rotational speed. Given the same twist rate, a bullet coming from a 7" barrel can't possibly spin as fast as one coming from a 20" barrel.

That's just how it seems to me.

It is 1 rotation in 7". Doesn't matter if it is going 3k FPS or 2K FPS it is still 1 rotation in 7". Speed has no bearing on the spin rate as it will be a constant 1 rotation in 7" coming out of the barrel no matter the speed.

Velocity and spin are two different things all together.

[mif_slim]

My 6.8SPC is 11:1 - So the 9:1 is probably the best compromise for 5.56 rounds....

those are some fast twist barrels you got there!

[Nathan Krynn]

Those are not fast.

You can't compare a .270 bullet to a .224 though.

[mif_slim]

Those are not fast.

You can't compare a .270 bullet to a .224 though.

those are some fast twist barrels you got there!

its fast... 11 full twist in 1 inch is really fast vs 1 full twist in 11 inches..