Yes, I know, slugs, but if you aren't on target perfectly then you're pretty much screwed. Am I missing something? I want to be good with the shotgun. :(

Um, well are you asking for tips on using buckshot or slugs? Pump action or semi-autos?
  
Pump action:
Slugs do 100 damage up close, 50 damage at a distance. Specing magnum ammo will let you kill body armored opponents up close and slightly extend the one hit kill range for others. At a distance a headshot will result in a one hit kill. A pump action shotgun with slugs is effectively a bolt action sniper rifle without a scope. The sniper rifles in the game seriously have very similar stats. You can Y-Y (switch weapons quickly) immediately after shooting to bypass the pump animation, something the bolt actions can't do. 
  
Semi-auto: 
With slugs they function the same as the semi-auto rifles in the game (M14, T88, etc.). If you don't take magnum ammo they will kill in 3 shots up close, 4 at a distance. Bringing magnum makes them kill in 3 everywhere (against non-armored infantry). I usually bring the extended mags or whatever it's called for these because I tend to get a little bit spammy. The semi-autos are much more forgiving than the pump actions. 
 
With buckshot? The principal is the same for semi-autos and pump actions. Get up close. The semi-autos fire exactly half the pellets the pump actions do and can't kill in one hit up close unless you aim for the head (and headshots count!). The reticle is deceptive. Always put your target dead center in it. 
 
The 40mm shotgun will surprise you.

I used to run with semi-auto shotguns for a while. Buckshot, not slugs.
You have to know what maps to use it on and what parts of the map. Constantly on the move in buildings and between cover.
Conquest maps are more suited to CQC then Rush in general.  (one example would be conquest Arica Harbour)
Run as recon, with use of motion sensors you can become a CQC killing machine. 

Don't really see the point of using slugs on shotguns. If you want to do that either use a sniper rifle or an assault rifle. Otherwise it's just for lulz.

Slugs+Magnum on a shotgun allows me to snipe brokingly accurately at any pixel flicker in the distance and not be entirely screwed in closer combat, but it's more for the open Vietnam maps than regular BC2 where buckshot will take you places, especially in Rush maps.
 
Of course, this is from a PC perspective, completely different game :/
 
The way to do it on PC is to master fast reaction times but without moving your aim too far, to be able to flick the mouse accurately and squeeze off a shot at the right moment. I'm just surprised I haven't received any hacking accusations in Vietnam this far with 870 Slugs+Manum.

" I used to run with semi-auto shotguns for a while. Buckshot, not slugs. You have to know what maps to use it on and what parts of the map. Constantly on the move in buildings and between cover. Conquest maps are more suited to CQC then Rush in general.  (one example would be conquest Arica Harbour) Run as recon, with use of motion sensors you can become a CQC killing machine.  Don't really see the point of using slugs on shotguns. If you want to do that either use a sniper rifle or an assault rifle. Otherwise it's just for lulz. "