Ar15 build questions I've never really been an AR guy, but I do recognize the ARs abilities, as well as being more economically friendly, which is exactly what I'm looking for, a fun winter gun project that's mostly within my "budget." My basic overall goal is to have a short entry stock, flat top for future optic, free float forearm that covers most of the barrel. I'm not real picky on who manufactures the parts, like I said "economically friendly." I'm mostly looking at buying or trading into a complete upper. With that said, onto the questions: 1) if I buy an upper barrel assembly with a standard front sight post, is it fairly simple to change it to a low profile gas block? 2) If I buy an upper with a low profile gas block with a handguard retainer, does the retainer remove from the gas block easilly (or at all)? 3) Ive been finding some sites with various parts for sale, but is there kind of a catch-all website that you recommend that has pretty reasonable prices? 4) I googled this but I just want to clarify, I can run a 20" upper on a carbine size buffer and buffer tube and spring correct?