this is a model of the Little David Heavy mortar designed to be used during the invasion of Japan during WWII. It has a calibre of 36in and designed to break through bunkers and stationary defenses having to have its base buried so it could handle the recoil.
the vehicle used is a t-35 by HAPPYBATTLESHEEPhttps://www.thingiverse.com/HappyBattleSheep/designs