The correct way for adding weapons & magazines I believe is that you first add the magazines and then the weapon. If you do it vise versa the guy needs to reload to make the magazines available.
So in this case the problem might be just so simple. Try to make the poor guy to reload after adding the mag with
unitname action ["reload",stringname for the weapon] (or something like that, check command reference for exact syntax).
If you can make the reload to happen you should maybe first remove the weapon and then add the mag and then add the weapon again.. which might look a bit funny but should work.
I'm talking only in theory here because I'm not at my home computer, but check it out and let me know m'kay