This is the only melee weapon in the game that can be picked up. The Chainsaw, obviously, is only effective at melee range but can take out multiple enemies with a single swing. It is probably gasoline-powered as the damage and swings or charges limited, the capacity can be increased will skill of certain classes.
By zooming in with the mini zoom (Right-click on PC) you can continuously swing the chainsaw with relative quickness. When mini zooming without swinging, it creates a small but extremely powerful hitbox in front where the blade is as it revs up.