Some guy who belongs to North Grafton Fish, Game & Bird Club brought one of those to my Lodge's Turkey Shoot. Used it, too. Didn't seem terribly reliable; apparently you have to adjust it for the particular ammunition (and it wasn't adjusted right that day), or so he explained it to me.
Yup, that's what the ring is for at the beginning of the barrel. You are supposed to be able to tune the gun to specific loads by turning the ring which adjusts the amount of gas available to drive the piston.