Here's the thing about Saltzpyre's repeater pistol, unless it got changed in a recent patch resulting in massive riots...
It actually is AP, despite that not being reflected in the stat screen. An alt-fire blast at close range will instantly annihilate a stormvermin up to the highest difficulty levels, and will continue one-shotting them after with proper traits. That's why people use the repeater, and why it's actually one of the most-used guns for him on Nightmare and Cataclysm.
Rapier is great fun and fast, but I think "The meta" calls for his twohander instead. Rapier is perfectly fine though, even on higher difficulties, provided you get the right traits and can reliably poke stormvermin eyes with the thrust (yes, melee weapons can headshot too).
When you get a pair of drakefire pistols for Bardin, try him out some more too. Turns him into a miniature Sierra, with infinite ammo and a heat cooldown. Also, the alt-fire cone of death doesn't hit friendlies, hehe.
Hm, interesting. I could have sworn that my first attempt with the repeater pistol was an abject failure, completely ineffective against anything with armor (and everything else as well - seriously, it was a green-rank item and even trash mobs took two hits to kill more often than not), but I'll give it another go next time I pick one up.
And yes, I'm aware that melee weapons can score headshots. I actually have quite a lot of fun "dueling" stormvermin, stabbing them in the face repeatedly while avoiding or blocking their counterstrikes. Of course, the damn dwarf usually swoops in and steals my kill with his two-handed hammer, but if he's busy I can take out quite a few stormvermin all by myself with the rapier!