Actually, the more important question is what is the practical usage of the Second Amendment? If you think the Second Amendment is about duck hunting, preventing break-ins and plinking at targets, then the .50 BMG is probably more gun than anyone would ever need. If, on the other hand, you think the Second Amendment is about an armed citizenry being the ultimate check on tyrannical government, then a .50 BMG in private hands probably would strike you as a heck of a good idea.
A .50 BMG has a very practical military usage as a sniper or counter sniper rifle. Aside from that, it is a heck of a lot of fun to shoot, in the same way a 1200 cc Kawasaki sport-bike is a heck of a lot of fun to ride. The .50 is not a really useful hunting gun, but then the Kaw is not really a useful form of transportation, either.
Unless you are hunting Rhino or Elephant, or the Moose that bit your sister.