I loaned a friend some money against an ar-15 upper with the thought he would retrieve it. He didn't so I figured I might as well get a lower and finish it. Then he tells me it can't be a rifle because the barrel is too short, so I need to make it a pistol. Is it worth the hassle to have an ar pistol and if so recommendations on which lower to buy. Do I reload to same specs as rifle? Thanks in advance.