Cant say I have ever heard commercial or milspec referenced with the full length tube. I thought it only applied to collapsible models. Anyhow you are not limited to Armalite. You can use DPMS spring and rifle buffer,DPMS rifle buffer and a Superior Flatwire Spring, SOG Buffer. Theres other options as well Brownells has the tube alone very cheap here They also have everything you want in one package here