The Reserve Shooter is a community made weapon in Team Fortress 2. It was added during the Uber update. It is for the Pyro and Soldier.
Military scientists were at first dumbfounded by the request made by soldiers of the airbourne divisions to make a gun with with the same firepower in a smaller package. It's usually their job to put vastly more firepower in a package that's the same size or much bigger. Grudgingly the work was completed, and this was the result, a compact shotgun.