The Mobile Tank Cannon is an incredibly powerful and automated reloading gun. It is two handed and fires a ton of buckshot at an enemy, causing massive damage but a hefty reload time. It is one of the rarest drops in the game, nearly on par with the black skulled weapons and many other artifacts of worthy value.
The Mobile Tank Cannon can only be dropped from Mechanical enemies, in the category of Tanks. The chances of getting a drop from these foes are 1.5%. Getting this drop is a huge achievement for many players, and is very valuable on the market. Mechanical elites increase the drop to 2%. Some players however have gotten over 600 mechanical kills and not received one.
It can be also dropped from a staked duel or the Nether Realm FFA, or being traded.