Buy a 10mm, and a conversion barrel. Easy peasy .
The barrels aren't that easy to come by, and the dies are very expensive. I have two sets of the Dillon Carbide 9x25 Dillon dies.
I had to have a 9x25 Dillon chamber reamer made, and then rechambered a .38 Super barrel for one of my Witnesses. The other one is a comped 1911.
Hope this helps.
Fred
At first I thought the post was ridicules. Now I'm curious about the .357 necked down from a 10MM. Any ballistics on that from a 5" barrel.
I pretty much stick with 115 gr. bullets in my 9x25 Dillon pistols, since it works so well. Without stressing anything, I'm getting 1,537 fps with it from a 4.5" barrel in a Witness. There was room in the data to go higher, but I didn't feel the need. I have the original notes from when Dillon was making up loading data, and I used that for my workups, settling on Accurate #7 powder with the 115 gr. bullets.
I did manage to get 1,837 fps with a cast linotype 103 gr. bullet and a powder charge I worked up from Quick Load. That was in my comped 5" 1911, but the second shot blew my comp off the end of the barrel, so I didn't shoot anymore of those loads.
I haven't really messed with the caliber much the last couple of years, since my interests went off in other directions.
Hope this helps.
Fred