Note that when you compare the 12.1mm to the 12.05mm you are comparing apples to oranges. The 12.1 diameter is over the "Rim" of the rimless 7x57 while the 12.05 is ahead of the rim on the 7x57R round. If you look at the diameter ahead of the extractor groove on the 7x57 it is only 12.01mm. Therefore the 7x57 just ahead of the extractor groove is .0016" "SMALLER" than the 7x57R just ahead of the rim. The 7x57R appears to be given a few tenths of mm's clearance at the shoulder the 7x57 doesn't have as the R round doesn't headspace on the shoulder. It is also my understanding the break open guns are compatible with the pressures of the 7x57 loads which were standard for many years & deemed suitable for the older Spanish M93 Mausers. I would not feed it higher pressure loads designed for "Modern" 7x57's regardless of which case they were loaded in.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra