They use a sabot to seal the "payload" from the propellent gas, and have a "stripper" that stops the sabot, but not the payload, be it chicken or foam. Although in the case of the foam, I think, but am not certain, that they put on a new tube that was the shape they wanted. Pictures show it basically square in cross section. I don't know how they "valve" the compressed gas into the barrel behind the sabot, possibly by puncturing a diaphram, as I've read that's the way some similar "guns" do it.