I'm sure it has been proofed in its life for Nitro loads & modern loads will be Ok .

It sounds like it may have been rebarreled with a take off ex Mill barrel, unusually for it to be Metford rifling but it might be, I think you should have it checked before shooting it if you are not up on the working of the older 303 rifles !

Crosses normally are from sold out of service marks !

Nice rifle to .