Most Finnish Mosin Nagants, except the M39, have .309 barrels. They can be slugged, using soft lead.
I rebuild rifles with new barrels, or buy aftermarket barrels from manufacturers like Match Grade Machine and Shilen. The picture below is a Thompson Center Pro Hunter with a 6.5 Grendel barrel, but the other barrels look identical to this. The barrels are .308 diameter. I asked MGM to make me a 303 British using a .308 barrel blank.
I had a gunsmith in BC rebarrel one of my No 4 Lee Enfields to 30-303, using a Shilen barrel. A 303 British with a 308 barrel. It looks like a regular issue rifle, but shoots easy to find, less expensive 308 bullets.