You might want to consider the .25-35 Winchester (6.5X52R) or the 6X52R Brettschneider (5.6X52R opened to 6mm). Both are relatively low pressure cartridges. The Brettschnieder was developed with insert barrels in mind. The brass for both cartridges can be formed from .30-30 Winchester cases.

I have a chambering reamer for the Brettschneider cartridge, which I would be glad to lend to you, assuming there would be no problems in mailing it over and back.