JDW:
I'd want some ground truth on it, i.e., actual measurements and try some of the 65mm cartridges that are out there. I measured a few diameters and you are correct that the average floats arond 0.740". I've seen some very tight German chambers but none that wouldn't accept production cartridges. Now this is with the exception of early 10 bores with current RST production where the brass seems just a tad large. I wouldn't consider opening the chambers at all though.

What's the weight stamp of the barrels? 1.278 Kg?

Kind Regards,

Raimey
rse