The whole reason 300blk came to market for for suppressed sbr's, the peak efficiency of the cartridge is with an 8" barrel and a 30cal suppressor and subsonic 220g ammo.
It matches very closely to the 7.62x39 ballistics with the efficiency of the 6.8spc but is still the middle of the road between the two being jack of all but master of none.
It is a very expensive caliber to have and shoot. You would be much better versed to get a 5.56 ar15 or 5.45 / 7.62 ak.
If ammo cost is a factor go with the 5.45x39 ak74, if you want best parts avaiablity for the platform go with the 5.56x45 ar15. if you are dead set on a 7.62 platform get an ak47 in 7.62x39.