Alpha Shooting Sports M4 Upper
look under citadel barrels. bolts are a bit more difficult.
i get my 7.62x39 bolts from m1 sales and JSE on the 7.62 and the 5.45-
you will likely have to trim the bolt a little and make some adjustments on the firing pin for the military surplus ammo.
you need to stop listening to me or you end up with a semi truck load of the EBRs!