This PolarStar Universal Gas Stock 13ci Tank Stock allows you to retain the likeness and function of a traditional adjustable AR-15 stock, but houses a 13/3000 air tank so that your air is on board your rifle vs an external air rig setup.  This stock is made by G&P for the R3 rifle setup, but is a perfect match for the PolarStar UGS.

PolarStar UGS 13ci Tank R3 Stock