Ammo is probably to blame. I shoot S&B laquered steel-case ammo all the time(cheap and accurate) and have never had even a slight problem with it. Oly arms uses SAAMI specs for their chambers, but Bushmaster goes by 5.56x45mm dimensions, so I doubt it's the rifle's fault. A Bushmaster (or Colt) chamber should accept just about any ammo you put in it, as long as it's good quality and in spec. Is the barrel chrome-lined? If not, it could be a barrel chambered to SAAMI specs, in which case mil-spec ammo may be too tight in it.