Small health-bar-things next to the weapons when you have them in your inventory. They would lose a point of durability every time a projectile was fired. There would be an admin menu where the admins could write how much dirability an item could have. Ie a pistol might have 100 durability, whereas a machine gun might have 400 because of the faster projectile rate. You could base the amount of loss off of rotondos old zombie mod ammo thingy. That said, you could make weapons like miniguns and flamethrowers have an extremely high durability, as they spit ammunition very quickly.
And there could be events like:
onactivate>player>giveweapondura>weapon>5
Or otherwise, this would just be the standard give weapon, the number being the amount of durability out of 5.
Ie, if the number were a 3, the durability would be half, and so forth.
onactivate>player>repairdura>3
In this one, the number is the weapon slot. so say I had my inventory set up like this:
Bow
Gun
RPG
Sword
Push Broom
The rpg would get repaired if the number were 3.
I think this would help in servers that had an economy, where not only did you have to get enough money to buy the weapon, but you lose it if you cant maintain it. You could also set the durability to one to make any weapon a single use. There could also be some sort of toggle on whether or not the item dissapears or just breaks.