Conditions for the Confederacy of Crunches achievement

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).

To your first question:

• Equipment is OK. So you can use pipe bombs, molotovs and bile jars.
• The Pistol is NOT OK, you have to be careful not to use it if you get downed.

I do not know about starting a section over. You can read some guides for this achievement over at True Achievements (Guide is for 360, but should be the same for the PC version)

It can be said very simply: no bullets. You're allowed to use melee weapons (obviously), you can beat them away with the butt of your gun, you can beat open a gas can (if you have too much health, I guess) and you can use throwables (molotov for example). But as soon as you fire a pistol, you lose the campaign.

So, you have to do it for the entire campaign. If you use a pistol, you have to restart the campaign, from the beginning.