Gustav fired 48 rounds during the seige of Sevastopol before the barrel had to be removed for relining at Krupp (a spare was put in place) but the shelling was over and Gustav removed from the battle.
However, it had fired 250 test rounds during proving trials before being deployed to Sevastopol, so that makes it a total of 298 rounds fired before the barrel was spent.
Gustav's sister, Dora, was deployed to Stalingrad but there's no record of her ever firing a round in a combat zone.
Operation Pedestal 1942, Malta: 12 August: Air attacks: An Italian photograph of the convoy under attack. An Italian SM.79 torpedo bomber is in the right of the photograph.
Off all the pilots, those guys I have the utmost respect for. Anybody can crash a plane and die, but intentionally crash one and try to live IMHO takes a lot of guts.