The Terrible Shotgun is a unique shotgun in Fallout 3.
The Terrible Shotgun is a unique variant of the combat shotgun and has the second highest damage of all weapons in the Small Guns category. The weapon has a far larger spread than normal shotguns, and full damage is only done if all pellets hit, making it most effective in close-quarters.
Critical hit damage with this weapon is actually applied individually on each pellet the weapon fires in a shot outside of V.A.T.S. If all nine pellets critical hit at the same time (such as in a point-blank sneak attack critical attack), the weapon's total critical damage is 9 x 40 =360. And with the Better Criticals perk, it can go up to 540. Add in the base 80 power of the weapon, and then double the damage because of the sneak attack bonus, and the weapon's total damage turns 1240, quite close to that of the Fat Man. In fact, if the damage is increased further with a head shot, this weapon's destructive power against a single target becomes even greater than the Fat Man, being able to kill even a super mutant behemoth with a single sneak attack critical head shot.
The Terrible Shotgun can fire a total of about 146 rounds, the equivalent of 13 reloads, from full condition before breaking. Even so, the weapon will not jam until 120 rounds are fired. Given that you do not reload until the current magazine is expired you will have 10 reloads before the gun begins to jam, but the gun will only have three chances to jam before it breaks.