syntax:

container addWeaponItemCargo [["weaponclass","silencerclass","pointerclass","opticsclass",["muzzle1magazineclass",ammocount],["muzzle2magazine",ammocount]], qty]

where the item part is in format of existing weaponsItems

so that it is possible to

container addWeaponItemCargo [weaponsItems player select 0, 1]

or

{container addWeaponItemCargo [_x, 1]} foreach weaponsItems player