Building The 90 Grain AR-15, Part Two

An overview of barrels needed in the heavy bullet .223/5.56 AR-15.

by Joe Carlos

Bullet, barrel, bedding. Back in the M14 days we figured that if you got these three things right you’d have a tack driver. The AR-15 has eliminated the bedding issue (kinda/sorta) so we can concentrate on the remaining two. In this installment we will talk about barrels and next time will cover bullets and other 90 grain loadings issues.

Last time we said that you will need a barrel with a 1:6.5” twist to shoot 90s optimally. There are barrels in 1:6 twist and 1:7 twist that are just on either side of that which should also shoot 90s. Of the three manufacturers of 90 grain bullets, Sierra and JLK recommend a 6.5 twist. Berger says a 1:7 twist will work with their bullets. I have tried a couple of different brands of 7 twist barrels with the Bergers and never had much luck. While the bullets stabilized and went in straight, groups were just too big for my liking.

These experiments were tried with 20” service rifle barrels. Perhaps in a 26” match rifle barrel where a person could get a more velocity (about 100 fps) the 7 twist would work. I have also tried 1:6 twist 20” barrels with the 90’s. The 6 twist worked better than I expected and a whole lot better than 7 twist. In fact, one of them is the best barrel I have ever tested in any twist rate and any bullet. There are disadvantages to going to a 6 twist, however. Barrel heating, fouling and pressure will worse. Let’s face it, it takes a lot to overcome the inertia of a 90 grain bullet in .223/5.56mm cartridges in the first place. Trying to get that bullet not only moving down the bore but rotating once every 6 or 6.5 inches is quite a chore. A 6 twist barrel only makes pressures worse and with no pay off with long 90 grain bullets, so you can imagine how much this over spins 77 grainers on the short lines.

Read more in our May 2014 issue. Back issues are available.

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