If you add magazines via addMagazineGlobal to a unit the unit will receive the magazines but if the magazine is empty, you will be unable to reload because it won't overwrite the full magazine with the empty one.
It seems to be a bug using the change firemode key, to grap the weapon, from the back.
Default handling:
http://teamspeak.fankservercdn.com/ArmA_3_04.16.2014_-_19.38.03.12_1.mp4
This issue (using the "change firemod" key):
http://teamspeak.fankservercdn.com/ArmA_3_04.16.2014_-_20.11.03.13_1.mp4