I see the OP noted it is a 9x57, not 8x57.
That is great, however you should still slug the bore with a pure lead slug, poke it out and measure the outside (major) diameter across the projections (from the grooves) to see what the dia. is. These were made from about .355" to .358".
If a .358" bullet will fit into a fired case, then you can use .358's for your loading. If the bullet does not fit, you will need to buy smaller dia. bullets for loading this rifle, or have a gun smith open up the neck with a chucking reamer or other reamer.