All .45 acp cases headspace on the case mouth. It does not matter if it is a wheelgun or a semi-auto. SAAMI specs determine this. Now some moonclips might hold the cases without them actually touching the chamber shoulder, but that is irrelavant. ALL rimless (not .22s) semi-auto handgun cases headspace on the case mouth. And I don't know of any die makers that make a "roll" crimp die for the .45 auto. It would be too easy for inexperienced reloaders to roll crimp their cases and try to fire them in a semi-auto. If a case was roll crimped and chambered in a semi-auto, and the firing pin managed to ignite the primer....Very high and dangerous chamber pressures would result, because the case would be wedged into the chamber and the case mouth would be jammed in past the shoulder. The die makers would be inundated with complaints from shooters, not to mention product liability lawsuits.. Maybe you are thinking of the .45 AR?...That case does headspace on the rim.