Those are gonna be 1/2 x 28, and 5.56 holes in them. Threading onto a .308 barrel will be hard, and the first round will be exciting...
Does that really matter, where you are? Pistols can always turn into rifles, but rifles can never turn into pistols. So says the federal law. I'll find the gas port data from my 13.5" Armalite build.
EDIT - Rene, 13.7" barrel will be better, because any company's "extended muzzle device" will reach 16". On the Armalite 13.5" barrel that I have, I used their own 4-port brake, which goes 16.10"...
Also, if you're building a pistol - only a pistol - barrel length won't matter much. Well, unless these assheads go through with all this pistol-brace shiit. At that point, make sure your gun is over 26" total. OAL.