A friend of mine has one that he wants to sell, it's black with the 11" hunter rail. Upper, lower, barrel nut, forearm part, and a bunch of little screws that hold it together (his words). Are there any special factors to watch out for with these upper/lower sets? Do I need a special wrench for the barrel nut, do they work better with DPMS vs Armalite barrel extensions, barrel profiles, etc? I don't want to purchase something, order a bunch of parts and then have a headache.