Take a .30-30 case, shorten to 10mm length, ream for 10mm bullet.
Peen the rim to increase its thickness and reduce its diameter.
Why would you need to increase rim thickness??? Turning down the rim to reduce diameter, I can see where that might be needed to fit 6 in a small (med size) cylinder. We're talking about a gun made for the ".40 Special", right? Not a converted 10mm. so there wouldn't have to be a space for moon clips that needed filling.
Also, not sure the ,30-30 is a good choice, to shorten and ream to .40cal.
Going by the drawings in my Hornady book, (and assuming I'm reading things right -do check for yourself), the .30-30 is a couple thousandths smaller at the case head than the 10mm is at the case mouth. And the case body tapers so its even smaller by the time you get from the head out to the point where you're going to make the new pistol case mouth.
SO, it seems to me that would require a multi step process to "blow the case body out straight" , and THEN ream the thick body brass down enough to take a 10mm bullet. And that's IF with the taper removed there is enough room to work with, because even straight the case would be about .002" too small in diameter to match book specs. AND add in the factor that the book specs are maximums and virtually all brass is slightly LESS than that, so its not impossible that the .30-30 case might actually be just a tiny bit too small in diameter to use in the ".40 Special" project.
A better starting point might be the .30-40 Krag case, its larger at the head than the .30-30 and still smaller in diameter there than the .30-06 family of cases. However, using the Krag case brings it own set of complications.
First , shortened to .998" (10mm case length) it would be a tapered round, not a straight body one. Not insurmountable, but I don't think that's what you're looking for.
Second, the large Krag rim would probably need to be turned down to reduce the diameter in order to get 6 (or even 5?) to fit in the cylinder.
And third, Krag brass isn't easy to find these days, and if Krag shooters find you cutting up their scarce (and expensive) rare cases, they would be very tempted to shoot you!
(someplace non lethal, like your butt, hopefully...)
It might be cheaper, and I think easier work, to get some like minded investors and put up the cash to order 100,000 (or whatever they require) from Starline to get them to make the case you want, new.
And then, of course, the cash to get the gun you want, made in that cartridge. Spendy start up costs no matter how you go about it, from what I can see. Upside, if you have the money to do it, you're in a position to name it, and get in the gun history books, and sell it to other interested folks.
I do not think there is a rimmed "parent" case that is ideal for what you want, but if you've got the $$$ you could have them made. If you've got enough $$$ you can get anything made.
