It's a Smith and Wesson 500 and it's large enough to be a gag prop on a Quentin Tarantino movie.
The bullets for this thing are 5 dollars a pop and worth every penny.
For those of you familiar with firearms, here's a comparison of bullet size for some common calibers. The .500 is in the middle.

I was first to fire the thing:
Notice that I wasn't quite prepared for the kick. I was wearing earplugs under those earmuffs and the sound was still impressive. The concussion from the blast sent my heart bouncing off my ribcage.
Because of the shotguns and the rifle we were using, they put us on the rifle range, so the people around us were firing some pretty heavy rounds, but when this gun went off, everyone stopped shooting and looked over, asking "What is that??"
At point blank range, the muzzle flare alone could knock over a moose.
When my buddy shot after me, he had much better control. Now the perspective in these videos don't do the gun justice. The size of the thing was immense.
Some Pics:

