By the way, I checked SAAMI spec drawings, and the .45 AR rim thickness tolerance range is 0.079" to 0.089" Therefore, just use 0.089" as the rim thickness in your calculation, as it should be the worst-case condition. I would have no idea what the protrusion of the .45 ACP head would be, as that measurement would be specific to Ruger. You'll have to measure that to get a good idea of a minimum value. A dial caliper should work, but a hole depth micrometer would be better.