This stat is most useful for comparing semi-automatic weapons like rifles, where one can see the full damage caused by a single shot, regardless of how long it takes to load and fire the next one. For most small guns, that is a single bullet, but for shotguns, it is a single shell (containing several small pellets), and for the Gatling laser it is a single beam.
and shows the damage caused by a single click of the mouse/trigger.
Damage: This information comes straight from the G.E.C.K.
