Ha! Like your BSW my Type B Mauser would probably mike around .323 for the first few cm too,
but unlike your BSW its solely due to throat erosion!!

Anyone looking down the bore of that Mauser would likely consider it a write-off, but it shoots one-hole groups off the bench at 50m with iron sights, probably due to the squeeze-bore effect!

A digression, but I am considering opening up the chamber-necks on a couple of the Lee Enfields in my collection with cordite-eroded throats, to shoot .318 8mm bullets swaged-down to ~.315, hoping for similar results!
Already bought the .318 bullets...