Not sure if its a function in ArmA. Might be able to have a way around to force add it after everything is done...
Currently the magazine in your gun is taken from the vest. Would be nice if you can have a "primaryWeaponMagazine" which forces the magazine into the weapon before doing the magazines in vest etc.
Especially annoying for Launchers, as you just can't put magazines in the launcher, so the ammo will be taken from the backpack load.
(ofc same for pistols too)