Successfully complete the indicated multiplayer weapon challenge to unlock the corresponding accessory in the changing room.

Accesory	How to unlock
Backwards Baseball Cap	Get 250 kills with the Shotgun (Burger Face III).
Baseball Cap	Get 75 kills with pipe bombs (Splatter King III).
Brain Sucker	Get 250 kills with the Enforcer (Laser Seeking Missiles III).
Canadian Flag Logo	Get 75 kills with the Garter Pistol (Tiny But Deadly III).
Cheese Hat	Get 250 kills with the Ripper (Swiss Cheese III).
Detonator Logo	Get 250 kills with the Rocket Launcher (Boom Goes The Weasel III).
Hoplite Helmet	Get 75 melee kills (Punchy McPuncher III).
I Shot Stupid Logo	Get 250 kills with the Railgun (Right In The Eye III).
Jester Hat	Get 75 kills with trip mines (Watch Your Step III).
Military Hat	Get 250 kills with the Pistol (Size Doesn't Matter III).
Retro Duke Logo	Get 250 kills with the Devestator (Meat Grinder III).
Ski Goggles	Execute a frozen player 75 times (Ice To See You III).
Tie-Dye Shirt	Get 250 kills with the AT laser (Medium Rare III).
Uncle Sam Hat	Stomp on 75 enemies (Pee Wee Vermin III).
Wolf Logo	Get 75 kills with Steroids (Roid Rage III).
WTF Logo	Get 75 kills with exploding barrels (Smile You Son Of A Bitch III).