I don't have much information on this, but it appears someone has finally put the 50BMG into a single shot pistol.
13" barrel and 12lbs. I'd like some more information on this, more out of curiosity than interest. I'm sure one shot would be enough to end the day, and the blast would probably loosen the rafters.

Birdman Weapon Systems

While the birdman gun is pure fiction there is an actual single shot 50 bmg. I have seen pictures of it and it being fired in AH back a few years. For some reason it was based on a 1911 grip frame with a large barrel on top. More than that I don't know. but it did exist.
Jim L