iirc, the main skill for Shotgunners, Fullbuster/Shotgun frenzy has a 3 second aftercast delay?
Even with Kiels that's pretty large. .-.
I suppose if that were implemented you could put two DDs in there for a DPS shotgun build. Since it does splash damage it might help in WoE when defending those tight corridors, you'd be able to AoE flinchlock people from range. Could combine that + mines for a defensive gunslinger setup. Although it really doesn't seem that viable... most gunslingers tend to stick to their Pistols and Rifles for DPS damage, and switch to the shotgun to use Shotgun Frenzy kind of like a finishing move to OHKO their opponent.
You can't use a shotgun as your main weapon, it limits you far too much.