okay so i was wanting to buy a stripped lower receiver for the AR and have some questions. if i wanted to get an upper for either an HK416/grendel 6.5/beowulf .50 i'd want to get a 5.56 lower and not a .223? are all lower receivers pretty much the same? or is there some brand that i should stay away from? if or whenever there is a "ban" on rifles and if i have an AR already built, than it would be grandfathered in right? does it have to be completely built or can i buy all parts after the ban since my lower was bought before the ban? ie. say you can't have a collapsible stock anymore will i still be able to get one if my lower was bought before a ban? does anyone have any links that i can go to and read more about these: HK416/ Grendel 6.5/ Beowulf .50 and any idea how much a complete upper for these would run around?