Yep, it's been done, but the problem isn't with the stock, it's with your stock tube: If I recall, the buffer tube on a real Armalite is larger (read: wider) than an airsoft one. There were kits a while ago which converted buffer tubes to real steel size, but I cant remember if they were only for the Systema PTW...
EDIT: Here is something which was posted a while ago and explains the problem:
http://www.airsoftcanada.com/showthread.php?t=663
__________________


Ár skal r?*sa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.