Ar15 1-14 twist rate

B

Bkims

Inactive
Hello gents. I need of a bit of input. I'm apologizing ahead of time for the long explanation, but I'd like to explain my thought process so you can all tell me where I'm wrong. (ok I means stupid lol)

I decided that an SBR would be my Christmas present to myself. Being that I like to do things myself, I am planning on building one from parts.

That being said, I have an issue with the efficiency of a 5.56 sbr. Apparently all those science people in lab coats say that if the bullet (5.56 specifically) is traveling less than 2400 fps, it looses that magic they call hydro-static shock and also won't tumble (yaw) reliably in soft tissue.

Reading up on my history a bit, the original M16 had a 1-14 twist rate. This made that 55gr bullet highly unstable and made it tumble almost immediately upon impact. In the arctic conditions the Air Force worked in, this made the weapon unacceptably inaccurate. So the twist rate was refined to 1-12, which had the unintended consequence of reduced the tumbling effect nearly 40%. This eventually was refined down to 1-7 by the time the m16a2 and M855 bullets were introduced.

The only accuracy I require out of this weapon is to hit a paper dinner plate at 100 meters. So accuracy isn't top priority here. I know 10 moa. Not asking much at all there. My top priority is making that bullet tumble reliably at short range without having to use space age bullets at matching prices.

I've already found a barrel maker that can make this barrel for me at a price not much more than a stock barrel. I just want to hear your input on the idea.

Thanks gents.
 
I would get a 9/1 twist
that will stablise most weight bullets
and it will still tumble in soft tissue
and much easier to find already made at a better price
 
Loss of velocity in the short barrel will reduce stability. Don't cut it too fine.
Look up the twist of the Viet Nam era CAR 15s with 10.5 and 11.5 inch barrels.
I would use nothing slower.
 
Get the 1 in 9 twist and shoot bullets in the 40-69 gr range. Don't limit yourself with a 1 in 14 twist. You won't be satisfied with it.
 
shorter barrel = lower velocity= faster twist rate required for any given bullet weight---so you're placing an extreme limitation on your potential accuracy (maybe even beyond "paper plate @ 100 yards capability) by looking at a 1-12 or 14 twist.
 
Modern bullets are way to good to worry about needing a bullet to yaw and tumble to be an effective stopper. My 11" sbr is 7 twist and i handload for it using either 62gr federal fusions or 65gr gameking. Both bullets expand very well even at low velocity.
 
The 7" twist is to be able to stabilize the M856 tracer round, which is significantly longer than either the 62 grain M855 or SS109 bullets. You don't need that unless you are planning to shoot those tracers.

Tumbling may be important if you are constrained by the Hague Accords to using FMJ bullets and want a work-around for them to be able to make temporary cavities as large as expanding bullets do, but without actually expanding. If you are not constrained by the Hague Accords, then get a twist rate that shoots well and use expanding bullets and forget about tumbling. I don't believe there is any advantage gain expected from having both tumbling and expanding.

My recommendation is you choose the bullet you want, estimate the velocity you can get from it, and then pick a twist that gives it a projected gyroscopic stability factor of about 1.5.

Do you have a bullet picked out yet?
 
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