I've come across an M-S rifle (possibly a model 1925) of a mystery chambering.

Its only visible barrel markings are British "7.62mm 20 ton BNP"and a couple crowns and a 4-digit serial number. It is a double trigger gun.

It was assumed by the owner to be chambered in 30-06, but the bolt doesn't come close to closing on a 30-06 factory cartridge. The muzzle swallows a 7mm bullet with no contact.

Any idea what chambering this is likely to be?