Because we didn't have one. Why the hell not?
General weapons and bullstuffNOTE: ALL WEAPONS RAYCAST UNLESS STATEDGauntletA whirling sawblade with an electrical charge to boot. It's a melee, I don't need to explain further.
Machine gunThrows a weak barrage of bullets downrange at a nice firerate.
ShotgunShoots a burst of boolets at about one shell per second. Accuracy? Don't bother using it at long range.
Grenade launcherLaunches grenades with a timed fuse. They don't travel that far, not counting the rolling afterwards.
Rocket launcherI think you all know what this infamous bad boy does.
Lightning gunBasically, it shoots a beam to where you're aiming. This beam damages. See it ingame for how it works, it's hard to explain.
RailgunThe uranium slug sniper rifle of legend. Shoots one instant-hit near-enough-instakill uranium slug at a bazillion MPH. (AKA raycasts)
Plasma gunLaunches a steady stream of plasma orbs in full auto. 'nuff said. If you jump and shoot down, it can propel you up a little.
The motherforgetin' BFG10KTake the plasma gun. lower the firerate a to 1 orb per second. Add some explosion to the orb, and you get this.
ChaingunTake the MG and boost the firerate and reduce the accuracy. Also, After firing it for a while, the firerate goes up EVEN MORE, and the accuracy goes so low, it can barley hit stuff.
NailgunShoots nails at the same firerate and tighter spread than the shotgun, with a bit more damage. The nails are projectiles (hurr) and also reflect off surfaces. Neat.
Proxy mine launcherLays mines that detonate when an enemy is near them.

Of course, there's the health powerups, armour and general powerups, but that's for later.