Are guns that fire .50 caliber rounds legal in the U.S. for civilian use?

So I was in a West Virginia gun store today and noticed a rather large rifle sitting on the shelf. Upon asking about it, the store owner claimed it was a .50 Caliber Rifle and completely legal across the U.S. I live in Maryland, and was curious whether that type of rifle and the round it fires is legal in this country and my state in particular (it looked like fun to shoot),