This would be the largest caliber gun in game, look at the pics below the size of the shell
. 1943 - German Sturmmorser Tiger (Sturmtiger) 38cm RW6-1 L/5.4
(no SdKfz No.) Assault rocket mortar on heavy
tank chassis
Armament: 1 - 38cm (15") Mirser StuM RW6 1 L/5.4 mortar
projector
1 - 7.92mm MG 34
Engine: Maybach, HL 230 P45, V-12, gas, 700 hp
Speed: 25 mph
Range; 75 miles
Crew: 5
Weight: 68 tons
This extremely rare vehicle was a SP 38cm mortar on a Tiger I chassis. Hull was unchanged and a large box superstructure was added. The short barreled mortar projector fired a massive 761 Ib. shell up to 18,000 feet. The design was unusual in that the projectile was rocket assisted and the gun tube had vents in the wall to expel propellant gases forward. An attached crane was used to breech load the projectile. It was intended for street warfare or operations against heavily fortified defenses. Interestingly, this weapon was developed in 1943 by the Navy
as an anti-submarine weapon. The Navy dropped the project and the Army mounted the weapon on the Tiger I Ausf B chassis as an assault gun. In December of 1944, 7 Sturmtigers participated in the Ardennes offensive but were thoroughly unsuited and unable to keep up with the Panzer IV and Panther tanks and, as such, made only a modest contribution. Some 18 Sturmmorser Tigers were produced in 1944 from damaged Tiger I tanks.

