“A Winchester City-Killer 12 gauge combat shotgun, bullpup variant. In excellent condition, it has the optional DesertWarfare environmental sealant modification for extra reliability.”— Fallout and Fallout 2 description
The City-Killer is a pretty powerful close-range weapon, outclassing pretty much every other small gun except the AK-112 loaded with JHP rounds. There are betteroptions for destroying enemies at close range, but the combat shotgun's ammunition type is far more common and efficient compared to 5mm or flamer fuel.
In Fallout 2, the combat shotgun is largely overshadowed by the superior H&K CAWS, and later by the Pancor Jackhammer, especially considering the slavers near Vault City carry CAWS shotguns much earlier than merchants or other enemies start carrying City-Killers.