There is a stock by PM Concept that accepts gas, although it appears to be in a CO2 powerlet sized tank stored in the grip... why its not in the buffer tube I do not understand..
http://www.redwolfairsoft.com/redwol...l?prodID=17378
It is expensive, but combined with
http://www.redwolfairsoft.com/redwolf/airsoft/ProductDetail?prodID=18802 rail it is the only way to make a copy of this now out of production kick ass shotty
Oddly enough it was a Canadian firm that made the stock and rail conversion for the real steel.