Players always ask "How do I add ammo?" with Virtual Arsenal.
In addition, there seems to be no simple way of adding other ammunition types, for instance as an Ammo Bearer role, if your firearm is an MX, how do you add LMG box magazines?
You cannot, without fiddling by changing weapon around.
Load arsenal.
Grab an MX rifle.
Attempt to add 7.62 magazines to your backpack.
In my opinion, all ammunition and items should be add-able to uniform/vest/backpack. Place the players primary weapon magazines top of the listbox + valid ammo types for that gun, then after that list the rest as well.
Though still problematic to the inattentive players, this issue will be somewhat addressed in a future patch. As of August 05 2016 dev branch notes:
bis_iceman: Added: An ability to fill uniforms, vests and backpacks with any magazine in the Virtual Arsenal (not just those compatible with currently selected weapons)
https://forums.bistudio.com/topic/140837-development-branch-changelog/page-42#entry3075108