Very interesting project. I have been toying with a similar wildcat I call the .405 Grenadier. It is based on the 7.62x54R case and topped with 300gr .411 diameter bullet. It was designed for the Marlin 1895 rifle. I had a Marlin rebarreled for the cartridge and it works out fine but I think it will do even better in a bolt rifle. I recently acquired a Siamese mauser and will be having it modified to test my theory.

This is the 7.62 next to the .405 Grenadier

This is the 45-70 next to the .405 Grenadier

By the way, the easiest way I have found to make cases is by just firing 7.62x54R cartridges in the .405 Grenadier rifle - fire forming.

Now, as to why I mention this here. I believe the .405 Grenadier would be a perfect cartridge for a modified Mosin Nagant. The extra magazine length would allow the use of 400gr Woodleigh bullets. I have the reamer and.......