So you're at chamber cast and COW fire form loads to find out what you have.
On mystery chambers like this I chamber cast first and then attempt to chamber a fired factory case that has been "smoked" dye-chemed, or sharpied. If all looks well at this point I'll assemble several Cream Of Wheat fireforming loads into cases that have had the upper two thirds annealed. I'll then use these cases , neck sized, without annealing, then fired a second time, to create sample cases for a die manufacturer.
Depending on the die manufacturer it will be a boring operation or they will create a reamer.
The recommenced full length sizing die dimensions for a particular chamber vary according to caliber, use, hearsay, opinion and voodoo.
I create the shoulder angle and radius on the cutting tool and adjust for taper on the compound.
For longer cartridges a taper attachment is needed. My eyes and tools are not up to building reamers and my one local option for custom reamers locally has retired. I stick to straight wall reamers and short actions now. I wish I could help but I haven't taken on work in 20 years.