One or the other of these pieces are out of spec., I could be a clown and suggest that the RRA lower has been milled to match the new super secret Military specifications, but I won't.
Suggest ya check the Bushy upper w/ a friends lower, (what-ever the make), if it matches up you'll know that it's the RRA lower which is out of spec. If the Bushy upper [b]fails[/b] to fit w/ another Bushmaster or Colt or DPMS lower than it's safe to assume that the upper was not manufactured by Bushmaster and you have a Franken upper......w/ a Bushmaster barrel.
Should it turn out that the lowers the problem:
Does this lower have a matching upper that fits w/ no problems?
If it doesn't, was this lower purchased stripped and built by you or a friend?
Depending on these answers, if material need to be removed on lower was minimal I'd remove it, if it needs alot of removal, I'd seek the advice of a trusted gunsmith.
Basically your first priority should be to determine WHICH half is out of spec....
Mike