Cellular core pipe is basically hard foam with a veneer of plastic on the outside and inside. It is commonly used for DWV (drain, waste, vent), and is not rated for pressure. Do not use it for pneumatic cannons. It can usually withstand the limited pressures created by a combustion cannon, but it is not recommended due to the fact that it is non-pressure rated pipe. It will have "cellular core - not for pressure" written on it, and probably DWV.