To know the exact cartridge, you should have the chambers casted. A good gunsmith can do this pretty cheaply. They take a cast of the chamber, and then measure it to find out what cartridge it takes. I would strongly reccomend this if you are not 100% positive what it is. You could mic the bbl at 7mm, but who is to say it IS 7X57 for sure and not rechambered to a different 7mm round. Good luck