Build and Shoot

About 2 years ago me and the other admins of my server somehow had infinite bullets/grenades.

I can't remember what command was used for the life of me, does anybody know how it was done?

You can replenish the inventory with the /heal command. There are some scripts that allow you to do this regularly, e.g. every time the player places a block. Unfortunately, it is not possible to do this without replenishing health too.