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Hi Andrew - I've had much success forming 6mm Rem, 7x57 and 8x57 brass into 9.3x57's as well as .270 and .30/06 cases to 9.3x57. 9.5x57 will be just as easy.

1st off, due to the excessive headspace in my rifle, I needed a longer body than is on the standard 8x57 necked up to 9.3.

This prompted me to neck the new RP 8x57's (and others) straight - then neck them down using the proper FL die, adjusted in the press to the correct height, to get a crush fit on chambering. By crush fit, I mean that when chambering the round for fireforming, there is considerable pressure on the bolt handle to lower it into position. This shows the case is just a tich (no more, no less)of interference to close the bolt. This ensures the brass fits perfectly in the rifle's chamber - BEFORE it is fired for the first time.

These are the dies I altered and made up for necking the 8x57 etc. and '06 etc. cases straight for the 9.sx57.

I use the same expander dies for necking .30/06 and .35 Whelen cases straight before necking back down to fit my .375/06IMP.

The first one is a .44 magnumb expander die with the expander sharpened a bit to fit smaller mouthed cases. The second one is an old shortened .300 magnum seating die with a "turned" expander button threaded to fit it's 10x32 main shaft. This die is only for expanding case mouths or these cases, so needs no other work as the cases clear it's interior sides.

These are 8x57RP new brass. The second from the left is an 8x57 simply necked up using 9.3x57 Fl die from Hornady, with it's perfectly smooth/polished tapered case mouth expanding plug. This case, when measured, had .019" headspace for my chamber - thus I lubed them inside the neck and necked them straight in one pass which is the 3rd case, then 4th case shows the straightened neck, necked back down using the 9.3x57 Hornady FL die.
I never lost a case doing this and they all formed perfectly - 1st time, every time. Note the FL die must be adjusted in the press to only form a new shoulder for a crush fit. Do not push the shoulder back further than needed.

This set of cases shows the same scenario for forming my .375/06 IMP cases, left to right.
1st- .30/06 case
2nd- .30/06 case necked straight in one pass
3rd -straightened case necked to hold a .375 bullet and give a crush fit in the chamber
4th -fireformed
5th -fireformed case with 270 TSX
6th -fireformed case with 300gr. Hornady RN Interbond

I will note here that I now use a set of Hornady .376 Styer dies for neck-sizing & seating bullets into my .375/06IMP cases.

Prior to changing to these lovely dies & for almost 2 decades, I used a set of dies I altered/cobbled from an old set of .300H&H dies using re-ground (green stone) concrete drills to bore the dies. I made both seating die and neck sizer dies fro them & only had to buy a seating stem. The bore job on the sizer die was perfect and did not need an expander button. I used a hand tool for decapping - a punch and hammer. So much easier with the 376 dies. I also had made a seating die with a sliding case neck alignment gizmo, just like the Hornady seating dies are now made.

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Daryl

"a gun without hammers is like a Spaniel without ears" King George V