Bayonet

A bayonet (from French baïonnette) is a knife, sword, or spike-shaped weapon designed to fit in, on, over or underneath the muzzle of a rifle turning the gun into a spear.

There is a range of weapons that spawn with a bayonet attached, the additional benefit of this is you can easily switch from firing your weapons ammunition to using the bayonet at press of a button without having to switch weapons.

The downside; if incorrectly select the fire mode you can make a lot of loud unintentional noise thus alerting all players and zombies nearby to your presence.