Im a AK guy so bear with me, Ok, so I got this complete RRA assembled AR lower in a trade. Problem is is that it is registered as a AR pistol. What can I do with this? Can I put a 16 inch barrel and a stock on it or do I have to build it as a pistol? I know that the majority of upper and lowers interchange so can I just put any upper on it that I want and it will fit? Also, on my oly arms AR it says "caliber multi". On this pistol lower it says "caliber 5.56". I really dont want to build a 5.56 AR. I would rather do a 5.45 AR or a 22lr or even a 410 shotty upper. Will this work with my lower?