There is a weapons and ammo listing in the Ed Depot.
Always add a magazine first, then the weapon. If you add the weapon first it will not be loaded when the mission starts.
Use the file description.ext if you want to offer the player gear selection in the briefing. Check out the Ed Depot - OFP file formats if you are not familiar with description.ext.
you can add/remove weapons to a solder, ammobox or vehicle in the same way.