All weapons in the game have durability. At 100% a weapon has its highest damage and sell price. Firing the weapon lowers its durability, and thus lowers its damage and sell price. At 0% the weapon will no longer fire, but remains in your inventory.

Anywhere that "N/A" is located means that this category is not available for this weapon.

The sign ">" means that I was never able to find this particular weapon at 100% durability in the game, and the number that follows is the highest I found for that weapon. So, if you were to find this weapon at 100% durability it will be higher than the number listed. All weapons with the ">" sign were found above 90%, so 100% will only be slightly higher.

There are two melee weapons in the game: Katana and Nailed Bat. The durability status is the same for them, but these weapons do not need ammunition to be used. Once their durability reaches 0%, they can no longer be used, but remain in your inventory.

P.S. This chart is a screenshot from a spreadsheet made with Apache OpenOffice.