Anyone done something like a 7mm bullet in a 30 Herrett case for a Martini ?

Roger

That sounds interesting -- or even a 6.5. A Contender in .30 Herrett was my first experience forming and loading for a wildcat.

Not a Herrett, but based on the 307 Win case. I have a 6.5 Bower. There was a 7,25 ... . I don't know if the Martini will be OK with the 307 case and potential pressures.
I think there was a 7 based in the 357 Max case - 7 International Rimmed (if memory serves).

7mm supermag is 357 maximum necked to 7mm with a very short neck