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.45 ACP Headspace   In contrast to the rimmed cartridge, the rimless cartridge has a case head which is of the same diameter as the case body.  As a result, some other means must be used to limit the forward travel of the cartridge when it enters the chamber.  With straight walled cases, such as a pistol case like the .45 ACP, the mouth of the case is left squared instead of being crimped into the bullet.  A shoulder is left at the forward edge of the chamber, and the mouth of the case abuts against this shoulder.  Since there is nothing for the rim to seat against, the traditional meaning of headspace ceased to have significance, and in order to determine the proper tolerance for cartridge seating, measurements must be taken from the bolt face to the shoulder at the front of the chamber.  For tradition's sake, this measurement is also called headspace.   Image Credit:  Hatcher, Julian S., Hatcher's Notebook, Stackpole Books (Harrisburg, Pennsylvania, 1966)  Page 235