I would like to get one for my 38-40. However, they don't list one for the 38-40. They do list one for the .40/10mm, Does anyone know if a 38-40 case would fit in a .40/10mm neck expanding "M" die?

....about that specific die, but the other M dies I have are all have the M die stem mounted in an over-sized die body, so as to accept a wide variety of cartridge cases. I would expect the 10mm die to be similar, and should work for .38-40 just fine.

CALL LYMAN