12g Buckshot

One weapon in the game is chambered in it, and every one of them fires a round that starts at 25 damage. What each weapon does with it differs: rate of fire and effective range change the answer more than the round does.

Base damage is before armor. Hollow point trades penetration for flesh damage and armor piercing does the opposite, so the round you load decides how much of a plate you get through. Every figure here is read from the game files.