my point of view: the 300 rook is not a high power cartridge

what would I do? turning the barrel out of the receiver, looking for a lathe with a four-jaw chuck for the octagonal barrel. I would drill the chamber area out, maybe to 10 mm diameter. than soft solder a short liner tube - 10 mm outside, 8 mm inside into the chamber.

looking for a reamer

there are simple 8,1 mm reamer for sale in germany,not expensive
just ream a cylindrical chamber with such a tool.