The Moose Valve fits in the blowpipe stock. It comes with a special tool that allows you to install the valve without removing or retying the stock. You simply insert the valve into the stock, then use the tool to expand the seal and tighten the valve inside the stock. The Moose Valve also acts as a stock style water trap.
It is important to empty the stock and let it air out after use. Allowing the valve to trap and hold water next to a wooden stock for long periods of tikme is likely to cause cracks in the stock.