I have two stripped DPMS receivers and plan to build two different configurations at some point in time. I am not in a rush since I already have a new Bushmaster XM15A2 20" target rifle. The question is that when I purchase an upper should I get it complete with bolt and carrier? My concern is that if I purchase the upper and then use a bolt and carrier of different make it would have to be headspaced professionally wouldn't it? I would like to get some of this stuff ironed out before I actually start. My builds will be flat tops so that I can mount scopes or other optics closer to the bore and they will both be rifle length barrels. That is my plan anyway unless my son wants to use one of the receivers to build an M4 type carbine.