I have been playing a lot with shotguns lately and I have found a particularly efective class with the MST-255. The class is listed below and has some flexibility.
Primar Weapon: MST-255 Shotgun
Attachments: Muzzle Break, Red Dot Sight
Secondary Weapon: Magnum Handgun
Attachment: Acog Scope
Perks: Ready Up, Reflex, Quick Draw, Sleight of Hand
Tactical: Concussion Grenade
Lethal: Throwing Knife
This loadout is differs significantly from traditional shotgun loadout, but I have found it very effective even on some larger maps. The MST-255 has a fast rate of fire and a 5 round magazine. It has more punch than the bulldog and slightly longer range. This shotgun load out relies on rushing up and ADS firing the weapon rather than flanking and using hipfire. The MST fires fast enough that I don't have to get a one shot kill to win a gun fight at close range all the time. The throwing knife gives me a close range option if I don't want to show up on the radar. However, I carry it mostly to deal with guard dogs. I will often find myself without enough shells to kill both the dog and the player and the knife kills the dog in a single throw. The concussion grenade is just useful filler since I didn't have another one point perk that helps the class out.
The magnum serves many purposes in this load out. It takes some practice to get the timing down, however if you let it settle between shots it's very accurate at mid to long range with the acog scope. It serves as a long range weapon and works very effectively against stationary targets if I see somebody outside of my shotguns reach. The weapon is more difficult to use up close with the acog on it, but since my primary is a shotgun this isn't much of a problem.
The perks are a litte different from a typical shotgun loadout, but they suit my needs well. Quickdraw let's me ADS both weapons quickly and makes the magnum more reactive. Ready up is self explanitory on a shotgun load out. Both of the guns suffer from a long reload so sleight of hand keeps me in the action faster. Reflex is the real lynch pin of this build. It lets me swap between the guns instantly depending on the range I need to react at. It also speeds up the throwing knife considerably and sends my concussnion out a little faster. However, the biggest help is that I can trade off either of the weapons for something off the ground it I want. The Shotgun has rather limited ammo and when it runs low I need to find a relplacement. Likewise, I will often swap the magnum out if I find a longer range weapon on the ground to my liking. Reflex let's me switch fast enough to be effective.
This loadout still relys on forcing closer engagements to be effective, but it is diverse and just works well for me. There is definately some room to experiment with it. Having, reflex, readyup and quickdraw mean that I have perks available for just about anything I pick up off the ground. I was running it with ammo crate, ballistic vest, and the squad mate but it would function well with any streaks.