currently going for diamond on the shotgun class.
I started with the KSG, because the 870 is boss and will probably be the easiest.
the KSG is an absolute fucking beast, the one shot kills are pretty much all you will get unless you're too far out. I highly recommend laser sight and long barrel. Fast mags allows you to load 2 shells at once when reloading, but with 14 in the tubes right off the bat, you won't have to reload much if at all before you get killed. The long barrel adds 15% range to every shotgun, so its one of those must haves, especially to increase your one shot kill distance. Hip fire the KSG, aiming down the sights makes your margin of error too small and you'll end up getting fewer kills, unless you're a fucking champ with accuracy.
I am now working on the S12 and its a real pile of shit, sure you can empty a 10 shot mag real fast, but you need to be real close for a 1 shot kill. The reloads are horribly slow. Fast mag is a big help. You really need to concentrate on aiming down the sights with this shotgun as that greatly reduces the spread of the pellets. I'm currently running long barrel and fast mags, hip fire is about worthless for a 1 shot kill unless you shove the barrel up someone's ass, so laser sight is a waste. Big S12 tip, play hardcore for the 250 one shot kills.
Wish I did the 870 before they nerfed it, but it still seems to be a very dominant gun and I should have no trouble getting it gold once I roll onto my next prestige and the other shotguns are locked.
EDIT: i also owe the executioner a bit of an apology, apparently it is a one shot kill up to about 5 meters, so I guess it can be a good close range secondary.