View Full Version : Barrel Length and Twist Rate
How much does barrel length come into play when choosing a twist rate?
I am very aware that twist rate and bullet weight are intertwined but would like to know where the length come into it.
Specifically, between 1/7 and 1/8, what would be the better twist for a 14.5" mid length gas system barrel in order to shoot 55gr to 77 gr. ammo?
https://www.m4carbine.net/showthread.php?756-Barrels-101
What's your conclusion from that thread?
How much does barrel length come into play when choosing a twist rate?
I am very aware that twist rate and bullet weight are intertwined but would like to know where the length come into it.
Specifically, between 1/7 and 1/8, what would be the better twist for a 14.5" mid length gas system barrel in order to shoot 55gr to 77 gr. ammo?
You'll be fine with either, but I'd choose 1/8. From what I've read, most 1/7" twist barrels are more like 1/7.7" twist anyway.
1/8 is plenty for any reasonable bullet. I used to think that barrel length had dick to do with twist rate. But I did a test that showed the same barrel loaded subsonic and supersonic with the same bullet yielded much different results. This was in .308.... I had like 5 or 6 MOA with bullets yawing through the paper at 100, then loaded the same bullets super the next week and they shot about 1 MOA. I'd have lost that bet. I guess velocity and RPM do indeed make a difference.
1/8 is plenty for any reasonable bullet. I used to think that barrel length had dick to do with twist rate. But I did a test that showed the same barrel loaded subsonic and supersonic with the same bullet yielded much different results. This was in .308.... I had like 5 or 6 MOA with bullets yawing through the paper at 100, then loaded the same bullets super the next week and they shot about 1 MOA. I'd have lost that bet. I guess velocity and RPM do indeed make a difference.
I have read and I don't recall where that the bullet should have two complete twists before it leaves the muzzle which is why the military designed the M4 with a 1/7 twist.
I have read and I don't recall where that the bullet should have two complete twists before it leaves the muzzle which is why the military designed the M4 with a 1/7 twist.
Interesting... sounds somewhat plausible. But a 1/7 twist 10" barrel doesn't get 2 complete revolutions before leaving the muzzle... fyi, the 1/7 twist was actually designed for the tracers, which are fairly long (longer than 77gr SMK's).
I firmly believe velocity and RPM are interchangeable (within reason). More velocity doesn't always require as fast of a twist to stabilize. YMMV of course.
vicious_cb
10-06-14, 23:22
How much does barrel length come into play when choosing a twist rate?
I am very aware that twist rate and bullet weight are intertwined but would like to know where the length come into it.
Specifically, between 1/7 and 1/8, what would be the better twist for a 14.5" mid length gas system barrel in order to shoot 55gr to 77 gr. ammo?
Start thinking in terms of bullet stability as a function of both twist rate and velocity. If you have a slower twist rate, lets say 1/9 twist and want to shoot 77gr bullets accurately, you would need a higher velocity to keep the bullet stable where as a 1/7 twist would keep the bullet stable at lower velocities. Which is why 77gr bullets work just fine out of 1/9 twist 20" barrels(at least out 100 yards) or longer but have terrible accuracy in 14.5" barrels to the point where the bullet holes look oblong.
Failure2Stop
10-06-14, 23:32
I consider 1/8 to be the optimal twist for 5.56, with 1/7 being the second best.
Start thinking in terms of bullet stability as a function of both twist rate and velocity. If you have a slower twist rate, lets say 1/9 twist and want to shoot 77gr bullets accurately, you would need a higher velocity to keep the bullet stable where as a 1/7 twist would keep the bullet stable at lower velocities. Which is why 77gr bullets work just fine out of 1/9 twist 20" barrels(at least out 100 yards) or longer but have terrible accuracy in 14.5" barrels to the point where the bullet holes look oblong.
My 16" 1/9 shoots 77gr SMK's amazing. Optimal? No. But it does stabilize them at 100yds. Further than that, I don't know...
I have read and I don't recall where that the bullet should have two complete twists before it leaves the muzzle which is why the military designed the M4 with a 1/7 twist.
Interesting. I've not found it to be an issue at our altitude however.
I consider 1/8 to be the optimal twist for 5.56, with 1/7 being the second best.
Agreed. 1/8 is really the answer.
Not even a single revolution is needed to fully stabilize the bullet. All that is required is enough rifling to impart spin to the projectile. Bullet stability is dependent on revolution per minute first and atmospheric conditions second. (A bullet fired in a vacuum can be of any shape and need no rifling at all)
Not even a single revolution is needed to fully stabilize the bullet. All that is required is enough rifling to impart spin to the projectile. Bullet stability is dependent on revolution per minute first and atmospheric conditions second. (A bullet fired in a vacuum can be of any shape and need no rifling at all)
Why would you shoot your vacuum? :p
Why would you shoot your vacuum? :p
No, no, no- It's not a bullet fired into a vacuum, it's a bullet fired in a vaccum. If there's enough carpet dirt in there for a good backstop, there should be no problem
MorphCross
10-07-14, 19:42
A bullet fired in a vacuum can be of any shape and need no rifling at all.
Weapons of interstellar terror. A mathematician with a pen, paper, and a firearm in Space. Also why no space agency should allow abandoned satellites to just float around Earth in a perpetual orbit. I digress.
The amount of bearing length a bullet experiences against a barrels rifling here on earth at certain atmospheric conditions will determine what twist is necessary for stabilization. 1:8 won't hurt in most circumstances. True 1:7 will stabilize bullets that won't even fit an AR mag C.O.A.L. when loaded.
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