There is absolutely no reason a civilian should not be allowed to purchase a .50 cal if they want to. They are used in long distance competition shooting and have been for years.
That's about the only reason I can think of where a civilian
should be allowed to purchase one. And, if that was the only justifiable reason for owning one, such weapons should be
very tightly controlled: background checks, permits, etc. And, owners would only be allowed to transport them to and from competitions and gunsmiths.
There is no big game that would require that weight of a round to bring down, nor should any hunter need the range that a .50 cal rifle would provide.
When have you ever heard of anyone being shot or held up by someone with a 50 cal BMG round.
I can still remember this robbery being in the news over 30 years ago. A quick google only gave me this. If had more time, I might be able to come up with some more details.
"Another more interesting robbery took place in Canada in Montreal in (I believe) 1976. There was a Brink's truck that covered all the major banks in downtown Montreal. It seems that every morning the driver and crew would stop for coffee (at the same place, same time), then drive to a major bank where the crew would take a freight elevator downstairs, then deliver and pick up their cargo. While this was taking place, the driver had a daily routine of drinking his coffee, while also reading his paper ! This was done of course by holding it up, and thus obscuring the truck's windshield.
One day while reading the paper, the driver hears a rap on the window, looks to his left , and sees a guy standing there, pointing ahead. The driver puts down his paper, looks up, and there sits a delivery van with the rear doors closed. When the doors are opened, there's another guy sitting behind a .50 caliber machine gun, mounted behind sand bags! Merde! Having been in the military, he knows that once that thing is fired, it's rounds will go through the truck like a hot knife through butter (they ain't armored enough to stop that). He gets out (the one thing you're warned never to do), waits for the crew to come up with their haul, and once they show up, the rest of the crew is forced to open the truck, then tied up. I'm a little fuzzy on the total amount they stole, but I believe it was around $20mm+. As far as I know, only a small portion was ever recovered, and that came many years later. FWIW, the .50 cal was not assembled correctly, and thus could not have been fired."