At the moment GroundWeaponHolder created with script will exist for a split second before it is deleted. This is a good feature to keep the game clean, but sometimes it could be undesirable as described in this thread:
The solution could be the introduction of a subclass _Scripted that has autodeletion disabled:
class GroundWeaponHolder_Scripted: GroundWeaponHolder
forceSupply = false;
This will allow mission maker to make permanent placeholders for weapons and magazines if needed and not worry about autodeletion.