All weapons do a random amount of damage. This prevents people from calculating exactly how many shots it will take to kill someone, because it simulates the erratic effects of combat. A bullet could just graze the target's skull without killing him, or a rifle round could go straight through the eyes or heart.
Random damage can also simulate the effects of underpowered or poorly loaded ammunition.
This is the weapon's name. Rifled Slug Winchester 12ga. Shrapnel Images and ideas from Fallout and Fallout 2 are the property of Interplay Productions. Half-Life and all associated images and ideas are the property of Valve Software.