Picked up a sporterized Mauser 98 which I assumed and was told was an 8x57. The custom German-made barrel is marked 7.8mm (not 7.9, 7.8, which I have never seen before). Can anyone enlighten me?

I would do a chamber cast. There are a number of variations in German 8 mm's.

It could be a .318 bore rifle. I would agree with a chamber casting and would also slug the barrel. I have had a few problems with German calibers and the nutty things Americans can do to them.

The 8x57JS and 8x57J Mausers were stamped according to barrel bore diameter as opposed to barrel at groove diameter. An 8x57JS would be stamped 7.9mm denoting a bore of 7.9mm (actually 7.89mm) and a groove diameter of 8.2mm. An 8x57J would be stamped 7.8mm denoting bore and groove diameters of 7.8mm and 8.07mm.



.318" barrels are all 8.07mm bore and 8.38mm groove diameters or pretty close to that and would generally have English proof marks of some sort.



What do the proof marks look like on your rifle?



The caliber possibilities are 8x57J or 8x64.



Addition:



A note on .318" calibers/bores. A .318" bore is usually 7.8mm bore and 8.07mm groove diameter or pretty close to it. Above I meant to convey that 318 Caliber (318 Rimless NE / 318 Westley Richards) are all 8.07mm bore and 8.38 groove diameter. Sorry about not being clear on that. This information is from the Lothar Walther website in the German section of the site.

I think Gina showed us all up with her superior knowledge of Mauser barrels and markings. My hat is off to her.

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I think it would be pretty hard to say what it is exactly without slugging the barrel and making a chamber cast. In the US it could have been converted into a 8mm-06 or it could be a 8x60 (some were .318 caliber).

I would try to make a determination quickly. If you have an oddball, it may be hard to return it to the dealer.

7.8mm uses .318 diameter bullets, and are avaiable from HAWK, and WOODLIEGH for two makers. A chamber cast, and barrel slugging should be done with any rifle you do not know the exact chambering for! NORMA used to make bullets in this diameter, but not today. I have about 2000 220 gr solids, and 1500 196 gr solt points made by NORMA, as I own several combo rifle/shotguns, and one double rifle that uses the .318 dia bullets! All mine are chambered for 8X57JR, a flanged, bottelnecked cartridge, which will push a 196 gr .318 bullet at around 2400/2500 fps.

Thanks to everyone for the valuable input. I will cast the chamber and slug the bore and report back.