Here's what I have done and it works in Crates, Ammo Trucks and PBR's.
First off, I make sure that all of them are empty.
Then I name them, example: Crate1, Ammo1, PBR1
Then in the Init field I type in this:
Crate1 AddWeaponCargo ["XMS",20]; Crate1 AddMagazineCargo ["m4",200]
I follow that with all the weapons I want to put in. Trust me, you screw that up and the editor will tell you. If you do it right, but screw up the ammo names, you'll know when you launch, cause it will desk top you.
What you might try is create a bogus mission that you can test new things in and put weapons crates and ammo trucks in it. Then you could perfect it and then copy those crates over to your maps without having to type in the scripting all the time. If your going to use all the new add-on weaps, you might switch to the new weapons add-on crates. I have used both Ammo Crate 1 and Ammo Crate 2 the same way, either one will hold the weapons.