He hasn't covered wars...
How To Unlock: Use every type of firearm on a zombie..
This achievement requires you to actually kill the zombie with the firearms. As all other multi-playthrough achievements, there have been reports of it not unlocking when it should. Again, try to contain the requirements in one playthrough, to be on the safe side.
■Assault Rifle - Can be bought in Pawn Shops, appearance seems to be random
■BFG - Combo Weapon, refer to ‘Duct Tape FTW’
■Handgun - Various locations
■LMG - Can be bought in Pawn Shops, appearance seems to be random
■Merc Rifle - Only found during storyline cases
■Shotgun - Weapon Shop in Pallisades Mall
■Six Shooter - Only obtainable after killing the psycho from WWJWD?
■Sniper Rifle - Weapon Shop in Pallisades Mall