I acquired a Bushmaster CMP XM15-E2S in the late 90's. It has a 20" Crome lined 8/1 twist 1" target barrel on it. Came with a weight to place inside the stock butt to balance the rifle. Problem is it weighs 14 lbs. I would like to buy a flat top upper for it. Questions: will my bolt fit another upper? Should I buy a complete upper? Should I buy a Bushmaster upper or what brand? And where? Should I just go ahead and buy a cheaper "carbon" rifle? I want something I can carry and plink with but want to be able to stick my target upper back on when I want. Any help appreciated. BTW I have a custom two-stage trigger in it that is awesome. Shot NRA High Power for several years (not lately).