Steam describes this achievement as:
Finish a campaign using only melee weapons.
Ok, so that obviously excludes any primary weapon, but what about throwables (pipe bombs, molotovs, bile jars) and the pistol that mysteriously appears in your hands when you're incapacitated?
Furthermore, if you accidentally fire a weapon during a particular chapter of the campaign, is the slate cleared if you die and start that section over?
Best Answer
Only melee weapons. So, any usage of any rifle or pistol (especially when you get incapacitated), including the grenade launcher will deny you of the achievement, even if you don't do any damage.
Chainsaw is a melee weapon, too, by the way.
Throwing molotov, pipe, bile jar or shoving zombies around (even killing them with a gun only by shoving) will have no negative effect. Also interesting is that usage of machine guns and stationary turrets won't cancel the achievement either, since they don't count as guns, but part of the environment.
HOWEVER, if you fire one bullet, you have to restart the campaign ALL over. Dying and repeating the map will not reset this (have actually tried it).