The .18 Derringer is a great little weapon for a character that wants to fight, -Just a bit-. Or for a character that's got something very expensive they want to play and need something to tide them over until the other thing fnds it's way to your hand.
Here are some unique cases for using this weapon:
A 1 or 2 fight character can resist the little things that may spawn on them, a clue focused Agnes Baker, Minh Thi Phan, Wendy Adams, Finn Edwards and others. Even if you attack foes at just a +1 over the difficulty the .18 Derringer is uniquely reliable because the ammo-rescue mechanic makes risky situations like "IF I hit these bullets, I wont need help." into "WHEN I hit these bullets, I'm out of trouble." The high +2 to hit bonus also means that these characters can guarantee a 2-damage hit on something nasty if needed (via Skills, Cornered, ETC).
A 3-4 fight character can use the .18 Derringer to tide them over until they get their actual main weapon and in that role, an early sidearm, it's just brilliant. There is a time in every scenario where a fighter character just hasnt played all the cards he need's to have in play to be effective, a time where a single dangerous foe like Yithian Observer or Agent of the King or Fang of Yig ETC could ruin your day, .18 Derringer is the weapon that stands strong without any support and beats all these things with ease.
The only notable point against using this weapon is the low ammo count for the cost, 2 shots might be all you need for the clue getter but a true fighter might still prefer an Enchanted Blade if it's available. Enchanted Blade and .18 Derringer together are a pretty sweet weapon lineup for a flex character.
Note that the upgrade is fantastic.