Probably a .700 Nitro Express or one of their 'Wildcat' versions. Real filling looseners!
Somewhere on that site is footage of a hang-fire with one of those cannons. The guy squeezes off the trigger, there's a pop then a whizz and a second later a God almighty explosion. The guy had started to remove his head off the stock to see what was wrong. It damn near killed him. Funny as fuck.
Edited to add- the rifle is I think chambered for '577 Tyrannosaur'.