Stay with at least a 16 in barrel. Look at some of the variations out there. Theres so much to chose from its near impossible for us to go over it all. But concentrate on getting a good barrel, good rails and good optics. Your upper receiver, as long as it fits the lower(which it should) you can relax a bit with. But your barrel should be 1-7 or 1-9 twist, chrome lined, with m-4 feed ramps.
This post was edited on 8/9 at 11:54 pm