Shotgun Guide

Shotguns are arguably the best weapon in the game, primarily because they can be used both offensively and defensively. I will describe both configurations below.

Offensive Approach
Offensive shotgunning is fairly simple, once you understand it's limitations. The shotgun overheats the fastest of any weapon, so you want to max out shots fired. I recommend the HWSG with 2 Frictionless Materials and either Shredder/Tungsten Rounds or any ammo you like except Hammerhead/Sledgehammer/Explosive. This will allow you to run up, shoot several people in the face, run back, lather, rinse, repeat. Good for any Combat oriented class.

Defensive Approach
Defensive is basically the opposite. This is for taking out enemies from behind cover, or taking out heavy charging enemies such as krogans or Geth Destroyers. Instead of maxing out shots fired, the focus is on damage. And lots of it. Fast. Recommend HWSG, 2 Scram Rails and Explosive Rounds. Explosive Rounds overheat the shotgun in one shot no matter how many heat sinking upgrades you have, so the Scram Rail's heat raiser make absolutely no difference on that factor. This configuration maximizes damage done, and combined with carnage, is capable of taking out most anything in one shot. Worst case scenario is that they fall over, leaving them open to a melee or two to finish them.