Tips for MP from a player that isn't the greatest shotgunner out there (getting better though).
Turn your sensitivity down, this will hurt you a bit in CQB with a shotgun but makes getting mid range hits with lancer and snub much easier especially on a quick moving target. You can gradually turn it back up as you get more comfortable with the gnasher, or just keep it low and learn to play around it like I have, you can still do well with a shotgun + low sensitivity.
You can be a big help to your team by staying a little back from the enemy and lancering while your shotgun equipped teammates push up. You force them to stay in cover and make it that much easier for glancing shotgun hits to become downs when you support with the lancer, you wont get as many kills but you might get less frustrated. Also chainsawing can work great to get easy kills as people round corners and due to the shotgun dominance people often don't expect it. When you are doing this though make sure to keep an eye on your flanks.
Play king of the hill, you can out a lot of pressure on the cap points by shooting in there with a lancer. You can also score a lot of point by breaking and holding caps.