For those of you who have never seen one, here's the head spacing problem with the old Colt Super 38 barrels. Due to the head space being controlled by that little rim on the cartridge, they would slip off of it and go further into the bbl when struck by the firing pin. Only the extractor would hold the cartridge in place. This caused lots of accuracy problems and gave the cartridge a bad reputation. Eventually, somebody figured it out and most newer barrels head space on the case mouth like a .45 or 9mm does. Old style on the left, newer after market replacement on the right.