I think there's a glitch where if you equip a shotgun with ext mags, it shoots more pellets per shot.
Yeah, it shoots 9 pellets with ext. mags compared to 6 without. It greatly increases your chances for a 1 shot kill with something like the Spas12 or KSG, especially when combined with the damage proficiency. On something like the striker, this is insane. It takes a while to rank the guns up though. It's level 26 for ext. mags and level 28 for damage and you will be cursing the game all the way there (unless you're using the striker) losing close range battles constantly.