I never thought much about putting different resistances on different PvP loadouts, since expected opposition is the same, but think you might be onto something. I'm sure a lot of the most dangerous opponents are paying attention to what you're most vulnerable to, so switching it up mid-game might prove pivotal.

I don't play much Lockdown, as I'm more of a Clockworks person, but for what little it's worth the Pepperbox is a bit of a head-scratcher. I haven't been to T3 yet, but I expect Fire status to become less important as damage and HP values are much higher, so a Blitz Needle's greater base damage might eclipse Fire. Human opponents are also much more dangerous than monsters AND better at backing off to heal, so up-front damage might be better in general than delayed damage anyway. I also don't know how effective Normal damage is in Lockdown, but have gotten a very strong impression that most players have weaker Pierce resist than Elemental- at least in T2 where I played the most. I've also read that Elemental resist is very overrepresented in LD, which corroborates this. (due to many people using Vog Cub armor, so it should apply to T3 too; If true, an Umbra Driver should perform better than Nova Driver, but be more risky with the Graviton Vortex in case of an opponent with high Shadow resist.) The Blitz Needle might then be a better fit for LD, though I do not know how important the pulling effect of Pepperbox is there. Switching to Umbra & Electron if you have them might be more efficient since you'd cover the same two types, Shock's movement disruption is a big deal, and the gun would more often be the ideal damage type. Shock seems to be a particularly PvP-oriented status effect; it stops us from moving, attacking, and even raising our shields. At least with Stun you still know how your animations are gonna work, but with Shock it's up to chance. That's what hurts me the most.

I remember thinking it looks like the Needle series has faster projectiles and a narrower spread than other Autoguns, but could not easily find a video or other confirmation just now, so take this with a grain of salt. ...an extra-large one, since I have a bias toward expecting needlers to shoot fast & light projectiles for high energy:momentum ratio, while the other Autoguns' graphics look to me like momentum-maximising pumpkin tossers. I even imagine the Needle series to produce less recoil, which seems like a type of thing I would mistakenly percieve for such reasons. OTOH, such things could well have been deliberately done by the developers since it would perfectly reflect the damage type...