Ah, I understand. So as long as it's pre-threaded milspec, it should fit and be headspaced correctly. If it is not, it's a factory mistake? I guess I can look up anywhere on the internet to figure out how to measure for headspacing though.
I'll have to look into getting a lathe sometime though.
Once again, thanks a lot guys. This morning I didn't know a damn thing about a barrel, now I feel like I at least grasp the basic concept.