I support the second ammendment, but before I can answer if the .50 cal BMG is a fine private citizen gun, I think that it's important to know it's practical usage.
If you support the the 2nd, you know that "practical usage" means militarily useful against any troops or indivuduals who are tyrannizing a free people. The least discussed but most critical words of the 2nd amendment are "the security of a free state."
A 50 BMG is important when the minutemen are facing redcoats who are coming over the ridge in an APC or other trucks, They work to put a hole in the truck's engine, so that the troops must evacuate in the wrong place, and get ambushed by other minute men around them.
Do you now see that the 2nd is not about hunting or target shooting (both of which build good riflery skills)? No, the 2nd amendment is about WAR. Rightful, moral, necessary WAR, waged by the people, to secure the freedom of their own state.