This is true. There is no gas tube and the barrell extension is different along with the twist rate and chamber size. http://palmettostatearmory.com/8923.php http://palmettostatearmory.com/2557.php http://www.karrisguns.com/ar-15-lower-receivers/assembled-ar-15-lower-no-stock By standard CMMG means the upper receiver hand guards, barrel nut gas block and flash hider. You would have to get a different barrel and a gas tube to be able to shoot 223/556 out of one of the dedicated .22 upper