Although not yet in the game, it is expected in future patches. It can be summoned on cheat servers by typing "mpcheat changeclass b" into the console. My extensive tests have concluded that this gun has about the same range or less than the pistol (50m), and fires a single slug. While effective against personnel (one shot kill), this weapon is meant for door breaching. It currently has a 7 round clip, has no sights, and is pump action. Info subject to change.
This is the maximum effective range of the gun (from the guy to the target): http://img56.photobucket.com/albums/v172/DesertFox_59/BreacherShot2.jpg
At longer ranges, the shotgun still has some effect, just minimal. At medium range, the gun requires you to empty the magazine, but will kill the target.
Do NOT switch to the alternate firing mode. It will lock up your weapon selections, and you will no longer be able to fire. It is not bugless, and was not implemented in America's Army: Operations (Firefight) v2.3
The shotgun has been removed from version 2.5., and is no longer in the game.