Your choice of barrel type will determine which type of bolt and gas system you must use. Armalite (AR10) and DPMS(SR25/LR308/etc etc) have their own proprietary bolt and gas systems, so it's practical to factor that into your barrel buying decision. I myself am very happy with an 18.5 mid-weight profile 1:10 twist barrel from Fulton Armory, it has done very well with everything I've run through it, but the heavier the bullet, the better it performs.