There is a function to do the opposite (Magazine owner of unit). Is there any way to get a unit group for all magazine units owned by a unit?
Ammo units have the hidden flag so you could maybe search in a region very close to the centre of the unit?
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I'm using an external magazine so that wouldn't be possible anyway.
I couldn't find one, but you can do it like this:
pick each (all units that can be a magazine ammo units in entire map) - if (Magazine owner of picked unit is what you need) then ...