Twist Rate faster on shorter barrel

[stubbicatt]

Saw a youtube video today by a guy who was shooting an AK74SU he had built for him. When he bought it, he said it had a standard 1 in 8" twist AK74 barrel on it in 5.45x39, which had been shortened to 8" from its normal 16 or 17" length. In the video he said that the short, 8" barrel, required a 1 in 6 inch twist to properly stabilize the bullets.

I do not know, but I guess rotational velocity is proportional to linear velocity, and is a function of linear velocity. Rotational velocity as rotations/time. As projectile linear velocity increases, so too would rotations/time.

I had never considered something like this before, that the shorter barrel would require a faster twist to adequately stabilize the projectile as rotations/time rather than rotations/distance than a longer barrel with the same cartridge.

 

[holdencm9]

You are right that the rotations are proportional to the velocity. 1 full twist every 8 inches, just divide the velocity in ips by the twist and you get the spins per second. So a .223 in a 8" twist going 3,000 fps (36,000 inches per second) will be spinning 4500 times per second. A round going twice as fast will spin twice as fast, for the same rifling. This effect compounds when shooting heavier bullets in the same barrel, because heavier = longer, which requires more spin to stabilize, but also means slower. Hence why they switched the 1:9 for 20" barrels in M-16's to 1:7 for the 16" M4....slightly slower V, but more importantly, heavier/longer bullets.

However, it probably only makes a difference when the barrel cut-down is huge. I imagine most bullets have a high tolerance of spin before stability is affected. For instance I you shoot anything from 50gr to 69gr in the same twist barrel and not have any issues, even though the heavier bullets spin slower, and are longer. Also, most rifle bullets get most of their acceleration in the first 10" or so. So it isn't really a "Sweet spot" of right amount of spin, as a sweet range of spin that will properly stabilize a bullet.

Ballistics by the inch doesn't have data for 5.45x39, but taking a look at their .223 data just for fun

55gr:
18" velocity is 2983 fps
12" velocity is 2724 fps (91% or 18" velocity)
8" velocity is 2380 fps (80% of 18" barrel velocity)

Seems to me like 80% of the original twist would still be enough for stability, but maybe he ended up having key-holing. I don't really know much abotu the ballistics of that round. Anyway, if you cut down a barrel that far, you aren't too concerned about long-range stability anyway, so probably not a big deal! But that is cool he got a custom twist to go with his custom build.

 

[helotaxi]

If the bullet isn't spinning fast enough when it exits the barrel, it won't be stable at any range. You'll notice keyholing at close range and erratic groups that open up exponentially as range increases. Not a good recipe for success.

The 5.45x39 was designed to be right at the edge of stability with the factory barrel length and twist. As such it doesn't take much reduction in velocity to get below the stability threshold. Also, since the 5.45x39 is a larger case with a (slightly) smaller bore compared to the .223, it is going to be more sensitive to barrel length with regard to velocity. It will lose more per inch of barrel length.

 

[stubbicatt]

Causing me to wonder if destabilization when a bullet goes transsonic may not be a corollary.

Anybody know whether rotational velocity bleeds at the same rate as linear velocity? i.e. what is it which causes destabilization of a projectile as it is slowing, and passing through the sound barrier again?

I'll just have to read more about this.

 

[helotaxi]

I was looking at a table somewhere where rotational decay was plotted. RPM decays extremely slowly compared to velocity. RPM was still around 96% IIRC when the bullet went transonic. Since the spin required for stability actually decreases with decreasing velocity, the bullet becomes "more stable" as its speed bleeds off. No amount of spin will help a bullet that has transonic stability issues.

 

[stubbicatt]

Thanks Helotaxi. I shall have to look into this further.

 

[holdencm9]

Thanks helo, I didn't know that about the 5.45 round being right on the edge of stabilization. I guess it would be the same thing with .223 if they had kept the old 1:14 twist and tried to shorten the barrels.

I wouldn't think there would be much resistance to the spinning so it makes sense to me that the rpm's would decay slower than the velocity.

I think the effects of the transonic range and the buffeting of the bullet and such are unrelated to the bullet spin, since it affects airplanes and such as well.