[2D] The DooM Slayers Super Shotgun
Yes it really is 2D although the world model is 3D for simplicity sake. This is really crudely implemented as when you look up, down or to the side it breaks the illusion.

Sourced from: L4D2 DOOM Shotgun
Half-Life 2 Mods Catalogue

Sourced from: L4D2 DOOM Shotgun