My Sauer drilling is almost identical to yours (side lever to open, top lever to select rifle), has a barrel that I have not slugged but it seems to be somewhere from 41 to 43. I shoot 2 1/2" .410 shot shells in the rifle barrel. They fit like a glove. Does a 2 1/2" .410 shell fit in yours? No one I have talked to suggests a 38-40 but in a Daly marked example, I would think that was a good guess. Mine is not Daly marked so I suspect it is a European metric caliber.