The brake Stag Arms uses on their 6.8 SPC has a 1/2-36 thread. It resembles a Miculek brake. It's steel and the machinng and finish are ok. The best part is that it's very inexpensive. Only $15.99 here:
http://www.riflegear.com/p-214-stag-...ator-68mm.aspx
Since it's made for a .277 bullet you should open the hole up to about .328 for the 308 bullet. A 21/64" dirill is 0.3281". Close enough. You'll want either a crush washer, a peel washer, or tweek it with a lathe to index it or it will look weird. I used my lath when I replace the A2 flash hider on my M1S Carbine with one. I just likeed the appearance. I agree with amafrank that it makes little difference. A 16" 300 Fireball has very little flash and not enough propellant mass to notice a change in recoil, at least not when shooting heavy subsoinics.