I got a good look at the thing when everybody else had left. It's a McMillan Bros action and stock, with a Krieger barrel. The scope is a Leupold 4x pistol scope. According to the owner Leupold is the only brand that holds up on his monster guns).
He said it's one of only three made -- the cartridge doesn't even have a name. The brass is custom made, bullets are lathe-turned from solid brass. I think he quoted $20/shot (the bullets are $6/ea).
Even with the brake, the thing wasn't too loud, though I thought he was coming off the bench every time he pulled the trigger.
You can see his "sidearm" in that last picture -- a Remington XP-100 bolt-action in 458 Win Mag. Watching him shoot that was even more impressive.
Truly, here is a man who "can beat his'n with your'n, and your'n with his'n."
jafager