Shotguns are OP as shit, high kill reward-- nearly no risk once you get up to Tier 2 with it's huge ammo capacity improvement, it's truely a hellish slug-puncher.
I've found fondness in a build where I use the rechargable critter with uber-super-criticals, all boomstick upgrades (and level 6) along with slow-mo perks, as damage numbers quickly hit up to 100 a singular pellet, along with a 66% chance for this to happen with every single pellet, it's a monsterous shredder of man.
Ammo Feed + Bouncing Betty + First Blood = Mayhem (I can see why this game wasn't supposed to be played online now, seriously you can place like 10 betties at once and be able to put down 3 more without even trying.)