Quote Originally Posted by 1903Collector View Post
However, as I understand it the differences are due to the physical point of the spec of each (USGI/Hatcher dims was measured to the beginning of the shoulder rather than a mid point of the shoulder
I don't think this difference in chamber dimensions came into being until the m1. The datum point is just a relative reference. If you check headspace of an m1 with its original barrel with different gauges, you will see the bolt is just a hairs-width or less from closing on a forester no-go gauge, maybe two hairs with a clymer no-go. On similar condition 1903's, with both these gauges the bolt falls way short of closing. I have confirmed this first hand. The USGI M1icon chamber was carefully designed to minimize failures under combat conditions and aid in the prevention of out of battery discharges (in conjunction with the design of the rear of the firing pin). The short version is that the chamber intentionally had some slop. CMPicon has an article explaining this too - and states the clymer gauge is the closest in spec that is commercially available.