I heard this article the other day. They said it's not that the ATF is afraid of gangsters getting the machines, it's everyday people and the risk they will impose on all society! Won't catch me firing ANYTHING expect a tennis ball through a plastic barrel. Barrels, firing pins and ammo are likely to stay metal... Plastics bullets, sure. Plastic casing and primers... Wtf?
PCP Ammunition - The Future of Ammunition