Every once in awhile, a revolver or shotgun will drop from stuff like chests, or enemies, or be in stores. Pick it up and it becomes an cn_X ability that fires a bullet in a straight line.
The gun does the same amount of damage as a bomb, so enemies that survive 2 bombs (like minigun guys) would take 2 hits to kill still. You can blow them up once, then shoot them to kill.
Just keep killing people with them across runs until you unlock this.