I had some ideas for a few weapons, and a couple modifications.
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Ideas
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Siren Grenade: Does the same damage as a normal grenade, except it also stuns zeds. Maybe give it to the berserker class?

Bloat gun: Shoots bloat vomit, kinda like the flamthrower. Leaves residue on the floor.

Insanity Grenade, or Berserker Genade: When thrown, makes the zeds fight each other for the time that the grenade is active.

Medic Shotgun: Normal shotgun, add medic darts shooter.

Throwing Knives: Need I say more? Def a berserker weapon.

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Modifications
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Sharpshooter: Give the xbow an alt fire. Such as an explosive arrow, or a poison arrow.

Fire Bug: Make the fire spread from zed to zed. (I.E. You are kiting a mass of zeds and you have a Mac-10. You shoot a zed and fire spreads from him to the zeds he's around.)

Grenade launchers have the same affect as Fire Bug grenades. The explosion is a big fire ball instead of an explosion.

Demo: Increased grenade radius

General: Give doors a shoot-through-able property. Wooden ones are easy to shoot through, while metal ones decrease the damage greatly.