I've been searching for a Browing engineering drawing for confirmation without success but will share this.
We know (certainly pre-WWII) Superposed typically had a 1/2" forcing cone to a bore of 18.4 mm = .724"
It has been stated that the chamber dimensions were slightly undersized. U.S. makers typically bored chambers .810 - .812" tapered to .796 - .798"
Do you have a micrometer?
I think Hal, Chantry & skeettx are correct that the expanding steel case heads are the issue.

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