Jason Jackson has received a patent (#8,176,837) for a AR-15 gas system that can be switched from piston-mode to direct impingement-mode by the operator. The piston itself is the gas tube. By adjusting the system, the piston can be locked in place and opened up to allow gas to flow through it into the bolt carrier.
I have racked by brain trying to think of a situation where a dual systems would be preferable over a just gas or just piston, but I can’t. It seems like needless complexity to me.
[ Many thanks to John for emailing us the link. ]Related