This is true...You should never use a case to check headsapce, why? Beacuse you can make a case do what ever you want. Depending on how the manufacturer sizes the cases (or the individual) the shoulder can be moved up or down so with a case I can make it work or not work. The gauge is fixed you cannot change the dimensions. If using the actual case was good enough they wouldn't be making headspace gauges. Builders would just buy a box of what-ever ammo and use them. The only quality control on our builds is "US". It would be like using a ruler instead of a digital caliper to take a measurement. Close just isn't good enough! Just my .02sks_hunter said:The headspace guage is solid, and the dimensions never change, giving you consistant tolerances. I only know this from handloading, and my own experiences with that, because when I've tried to set OAL for a specific round, this has happened to me. Although the cartridge headspaces on the shoulder, if the lead is out far enough, you may never get there. Just my $.02.