Only One Shotgun is a gameplay mod I made a few months ago that gives you as your only weapon a... well, a shotgun.
Though, the shotgun you're given at the start of the game isn't an ordinary pellet spreader. The shotgun you're given supports multiple, upgradable firemodes.
These firemodes are Direct/Hitscan and Splash/Explosive.
Direct firemode works like a normal shotgun at level 1-2, but at level 3 becomes a fully automatic firemode.
As you get the Splash firemode it shoots out impact grenades, at level 3-4 it will shoot rockets and at level 5 it fires powerful homing Mancubus fireballs.
But that's not it! There are also some additional features, such as the Hyperjump and the new power-ups.
Here's the release trailer. The download is in the description: (Let me point out that at the time of recording the gameplay I was terrible at aiming, but I don't fire like that anymore):
