Though not entirely true (the mod is a remake of an old game dating back from 1998-ish), this weapon concept is one of a kind. The following image might appear like just another noskinned weapon model to you. I will try to convince you otherwise. It's indeed a unique weapon.
Ok, so this is the concept:
It's 2066 on an post-apocalyptic Earth. The Outsider gangs are taking eachother out. You try to make a career in a gang called The Company, and all you carry around is this big heavy gun which appears to be the latest high-tech development. When you take it out for target practise you find out that reloading isn't necessary and overheating is not an issue. You take out some targets and notice a keypad on the side of the gun. Curiously you punch in a few digits and suddenly your rifle's barrels start shaking and twisting, and a sawblade pops out where there once was a machinegun. A bit stunned you take it out for a try on a sign with a ugly rival gang member's face on it. It cuts like a hot knife in butter. Punching a few other numbers makes the gun shake again, the sawblade folds up and disappears into the gun. Now where there was a machinegun first is a grenade launcher.
Basicly the rifle you have in this game is a multigun. It holds 10 different weapons inside which you can unlock with the keypad once you have obtained ammo for it. Reloading is not an issue, it's only the ammo consumption and the hordes of enemies coming at you. The story above was just a quick description of how it would look ingame. If you take a closer look at the image you can see the keypad. Also the bottom barrel has a set of hollow cylinders that slide over the barrel, altering it's function and slide back (enabling the chosen weapon).
Shame that this didn't make it to the front page.