I expect that the 50 BMG will be made a Class III weapon, because the Congress doesn't want to have to buy all of them back if they are banned outright. I also expect that it will only be a matter of days after that when someone comes up with a wild-cat, necked down 49 BMG.
What makes you think they'd honor the 5th amendment any more than the second. California isn't buying back any banned "assault weapons", AFAIK.
Besides, how can they "buy back" something they never owned?