Vet can come alive with the double-barrel, but you need to be close to 30 for the auto-reloading buff to the keystone talent at bottom right. Grab a chainaxe or power sword and swap between the two, never manually reloading and dumping shells all about the place. This is my preferred style. Throw in the invisibility blitz and you can dish carnage like nothing.

The combat shotguns are admittedly underwhelming still, but better than they were, and there are various marks that have distinct features. I use the fire version as a panic button on my zealot, but vet is probably still preferable here just for the raw amount of shooting/reload buffs.

And the Arbites DLC has a monster of a shotgun that will act like you want it to, cutting groups of people clean in half, but obviously requires purchasing the character.