RE: Which AR-15
HK45USP: "As far as .223's go, go with either an Armalite (Maker of the original AR-15's, before Colt), "
As a matter of trivia on true Armalite manufactured rifles, I fired a full-auto M16 in USAF basic training back in 1981, manufactured by Armalite, but marked as, "Armalite AR-15". Even way back then, it struck me as unusual.