Quick firing, but most lack the damage of other classes of guns. Useful silenced in a stealthy class or with double tap to fill the air with lead (P90 gives insane fire rate level).
The workhorse of the game and quite varied. Single and burst shot models can benefit from the stopping power perk to increase damage.
With these heavy hitting but inaccurate guns you want to be holding areas / buildings and using supressing fire across and into choke points.
The problem with shotguns is that their range is miniscule, in fact you could probably spit further. This coupled with the fact that you will have taken all targets down in the close vicinity in double quick time means you will find yourself without any targets in range. Using the RPG or frag grenades will give you some long range clout, or use the stun or flash grenades to give you the chance to rush to your next killing ground.
Long distance hidden shooting is the aim of the sniper. So after a kill you should ideally move to a new location. The hold breath should be used to assist taking the shot.