I like the Black Box with the Reserve Shooter and Disciplinary Action. Black Box is incredibly powerful for flanking and you get the Reserve Shooter for when you need an extra shot or two.
I also like the Airborne Armaments set. Shoot at feet, and as soon as you click, switch to the Reserve Shooter, and then kill them while they're in the air. The Liberty Launcher gets them into a "sweet spot" where you do max damage with your Reserve Shooter and its extra speed allows you to more reliably shoot feet. Even if you miss the feet, well... shotguns are really nice in general and people tend to jump around a lot anyway. And, of course, market gardening for fun and assassinations.