Currently we have a "Reloaded" Event that fires after a unit finishes reloading. Requesting one that fires as soon as the reloading sequence starts.
Currently, we are working around this using a [[ https://github.com/acemod/ACE3/blob/be77ef233ee87e00cca6e71a2ae307c6b073c6ca/addons/common/XEH_postInit.sqf#L439-L495 | mutex ]], featured in ACE.
In the upcoming 2.18 patch, the "MagazineUnloaded" Event has been created however, at this current moment, it only fires via unloading magazine via script or manually dragging the magazine from the gun. Please make sure to have it fire at the start of the reloading sequence as well.