Supposedly the ATF tried to convert this design (or parts from this design) to be used to fire 5.56mm ammunition like the real steal version, but they said it could not be "easily" done. Whereas previous models could supposedly be converted to a real steel firearm. I do not know how true this is, but that is how I understood it. Someone please correct me if I am wrong. Also, it was unclear as to what specific design changes were made, so yeah I guess someone will have to open this one up and take a peek inside to answer that one.