I am seriously considering buying one of the nicest prewar Mauser spotters I have ever seen the issue is that it is chambered in 9x57. Question 1 can I get away with using .358 diameter bullets in it ? Question 2 is there a simple direct rechamber ? It appears to be built on an intermediate length action so I am thinking the Whelen will not fit.

Maybe; slug the bore; many have 358 grooves.
No; for a Whelen you would create a little secondary shoulder. Not really a problem, just realize it.
Why rechamber at all; nothing wrong with 9x57 as is.
Intermediate actions have the same length mag boxes as standard 98, usually. I could never fathom why there were even developed; they are practically the same size a the standard 98 and accomplish nothing beneficial. Notwithstanding the safety breech feature on some of them, which, nowadays with our good brass, is not needed. I never fit them back: I fit barrels the standard way.
Lastly, if it is indeed, a nice, original Oberndorf sporter, definitely do not mess with it. You will only reduce its value.
If it is a private gunsmith build, then hack away; they are not worth anywhere near an original factory sporter.

Why in gods name would you want to change anything on it?

Already set-up for 9.1mm bullets (not the 9.3mm) or .35822", brass can be anything based on the 7x57

And easy to reload for.


Buy it, and leave it alone...

Phil

I have a guild made 98 mauser in 9x57.Barrel slugs about .3565".It shoots 220 gr Speer bullets on top of 3031 powder into groups under 1 inch at 100yds. Cases are easiest to make from 8x57 brass but 30-06 brass work real well but you have a bit of trimming to do.Sure lets a lot of blood out of a deer in a hurry !