IANAL, I've been over a bunch of the NH laws and none reference the .50 BMG. However finding a range to actually shot the thing is the tricky part in southern NH. I know they are not allowed at some ranges. I'm unaware of any ranges over 600 yards in the state yet.
If you acquire a .50 BMG I would like to witness you put some rounds through it. The range I go to would allow it, although it isn't nearly long enough at 100 yards. My friend is a Marine and has shot the .50 BMG several times, said there is nothing like it.