I remember one time in MW2 SnD I had my AA-12 (after I finally got tired of getting killed by **** with shotguns and started to use them myself) with extended mags and I killed 4 guys in less than 2 seconds LMAO, it was on Favela.
Just use the KSG. This game is pretty much about 50% latency and 50% skill. The KSG is insta-kill short-medium distance on normal, 2-4 long range (With long barrel). Also, use the toughness perk. With decent latency I usually have enough time to waste any SMGer that started shooting me thanks to toughness.
The only time this gun sucks is when the latency factors in and you shoot phantoms.
While I certainly would like if shotguns became as effective as they were in MW2 (but as primaries), the ones in Black Ops weren't terrible. The Stakeout and SPAS 12 were pretty well the best shotguns outside MW2 (since CoD4 anyway, dat Trench Gun, you don't know broken until you've used that thing).
I'm not completely disappointed in the shotguns in this game, the 870 at least feels pretty solid, almost like the Stakeout.