Extra shot, less cost. The perfect sidearm. The upgrade has a significantly less characterpool but those who can use it, love it.
Clue people, flex characters and money-starved combatants like Minh Thi Phan, Agnes Baker, Finn Edwards, Rita Young ETC can use this cheap piece of raw power to land unlikely hits and protect themselves extreemely efficiently. All of a suddent a random 3-fight enemy spawning on your clue-getter is'nt an immediately pressing issue.
Combatants like Tommy Muldoon, William Yorick, Silas Marsh ETC love thins thing too for being super-cheap compared to getting a whopping 3 shots at +2.
Note that the ammo generation combo with Live and Learn does not gain the inbuilt +1 to hit from the .18 Derringer ability, because the extra +1 to hit triggers the next time the action on .18 Derringer is triggered.