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Here's some extracts of what I wrote for New Zealand Guns and Hunting magazine some years ago about a 9x57 I then owned:

The classic load for this rifle, and the one that made it’s reputation, is a British Kynoch 245-grain bullet, (soft or solid, but always round-nosed), sent off at 2,150 feet per second. German ammo was more typically a 230-grain bullet traveling at 2,370 ft.p.s. for a muzzle energy of 2,860 ft/pds.

A close examination of the subject rifle showed it was a Mauser Model 98 action which had been reworked by Berlin Gunsmith Max G. Fischer. Gun Digest of both 1975 and 1955 has comprehensive lists of German proof marks. These show that the numbers 118.35 mean that this rifle was the 118th firearm proofed in 1935, so that’s the approximate year of manufacture. The markings on the receiver are also decoded: “2/8g GBP” means the rifle was proofed with 2.8 grains of Gewehr Blattchen Pulver or Rifle Flake Powder. “St. M. G.” means it’s suitable for Stahl Mantel Geschoss or Steel Jacketed Bullets. A crown over “N” means it’s nitro proved. Most German sporting rifles of this era have similar marks.

Getting these old metric rounds shooting usually starts with finding appropriate sized bullets. Often the barrel diameter is marked on the underside of the barrel, or the caliber on the side. Not so this rifle. There is nothing to indicate the caliber anywhere ! However, it was sold as a 9x57. This immediately creates problems as a 9mm is .356” whereas the nearest rifle size bullets available to kiwi reloaders are .358”. Using the larger bullet would create pressure problems which is not the smart thing to do with a 70-year-old rifle. However many of these older European barrels were under or oversize, with a 9mm being .354, .356 or .358”. The wise thing to do is to have a chamber casting made by an experienced gunsmith to find out what you’ve got. Another method is to take a slightly oversize soft lead slug, (I used a .38 caliber wadcutter bullet), and drive it down the barrel with a cleaning rod well wrapped with electrical tape to center it in the bore. I did the latter and a pair of slugs both popped out .358”. Just to be really sure I had a chamber cast done as well; the result being .358 ½”. Lucky me!

Interestingly a Mauser factory chart from 1925 shows that the normal 9x57 barrel grooves should be 9.08mm +0.05, (.358”). Of course, there’s no guarantee that Mauser actually supplied the barrel to Herr Fischer. This barrel does have German proof marks and, least anyone take such things for granted, a similar 9x57 rifle I sold for a widow had a Mauser action, a Belgian barrel and a famous English maker’s name.

If you have an undersized bore, you can look at swaging bullets to fit. A US company called “Z-Hat” seemed to me to offer the best deal in terms of a suitable swaging die. Their maker, Fred Zeglin wrote to me in 2000 that: “The ring die sells for US$65 for one caliber. Additional rings are US$9.50 each if you want to add other calibers lately”, (see http://www.z-hat.com ). Or you can put your .358” bullets on a centreless grinder. Or you could import correctly sized bullets from the USA. Or you could use a Lyman mould, (Barnes lists a 245 grain Lyman mould – 358318 GC), to make a suitable lead bullet.