There was considerable variation of chamber dimensions in 10G - both length and diameter, the latter most notably in the region of the chamber cone.
You have measured chamber length but should also take serial measurements at lower chamber, cone (and barrel) for comparison with your RMC shells.
Another option would be to make wax castings of both chambers and return these with the cases to RMC for alteration to fit your gun - or one particular gun. Your two guns may have different chamber dimensions - and each gun may have chamber variation in dimensions sufficient to allow or restrict their fit.
For optimum shell to chamber fit for thick walled brass cases the barrels should be sent to the maker of the hulls. Also best to stipulate a primer chamber to fit a 209 primer rather than the cap style primer.
If you have two different guns with different chamber dimensions you will need to segragate the hulls for each gun, they will fire form to the larger chamber dimensions. All RMC will come with 209 primer size unless you ask them for something else.
If you need to measure chambers a good quality choke gauge should work, if you want prefect then cast. In cast measuring chamber cerrosafe is the best for this.
