[MadMax]

I think redzaku is pointing out that the P90 has an unusual magazine arrangement for a typical GBBR because the magazine is on top of the rec'r with the valving on the bottom side as opposed to the top. Furthermore, the magazine has a horizontal orientation which makes it difficult to plumb the magazine to prevent liquid from coming out of the mag because slanting the gun would slosh the liquid around so there wouldn't be a defined place to draw gas phase from.

It is likely that propellant is conducted into an expansion chamber to provide reliable gas phase supply to the pneumatics which is a good and bad thing. On the one hand you could see more efficient usage of propellant because aerosolized propellant would be giving a large space to properly evaporate. This would improve full auto and long burst performance. A well designed expansion chamber might even allow sideways or inverted operation of the rifle! On the downside, a check valve would be needed between the magazine valve and the expansion chamber or you'd be depressurizing a large volume with every mag change. A check valve would prevent the escape of the expansion chamber volume with each mag change, but it would present a new safety issue with GBBRs. We are not used to gas powered guns being able to fire with their pressure supply (i.e. mags) removed. With a check valved expansion chamber, the gun would retain enough gas to fire a few shots, potentially several if liquid phase gas ends up in the expansion chamber.