|The following is based on Fallout Tactics and is not canon.|
The most devastating weapon of group attack. It destroys almost anything in one shot but the ammunition is very rare. Having to keep the tank in good condition makes this weapon hard to use continuously.
The main thing about this gun is its usage of driver's Big Gun skill. If the shooter shoots this cannon, the message will report that is the driver's name shooting. But if the shooter shoots their personal weapon, the message will report the shooter's name.
Uniquely for a big gun, the tank gun is classified as a long-range weapon, which greatly reduces the range penalty for firing, but imposes an additional aiming penalty for shooting at a point-blank range -- which is a bad idea anyway, as the tank shells explode! Additionaly, it's one of the few weapons that can benefit from the Sniper perk. 
On the tank, which is located in Newton, used only by the second passenger.