Seeing a Churchill AVRE just unexist tank traps and hedgehogs would probably be a scary yet exciting thing to watch... minus the whole shoving the explosive into the mortar-thing
The largest caliber weapon installed on a tank, a 290mm mortar. It's bigger than the FV4005 armament, but that is an actual gun. Mortars technically aren't guns, so it's the largest caliber weapon for a tank, just not the largest gun installed in a tank. And before anyone says, "Karl Gerat has a bigger weapon!" That isn't a tank. It's self propelled artillery. Nerdy rant over. Edit: Oh, and the sturmtiger is a self propelled gun. So it doesn't count either.