From Johan in the Comment section of Random Curiosity
"I am curious about the caliber choice of 9 mm. I would think that .38 special/.357 magnum chambering would be easier for production with a minimum amount of equipment."
I see two advantages to the 9mm Luger for this application.
Mass of the platform
The SAAMI maximum over-all cartridge length for the 9mm is 1.169". Add an additional 0.10" for clearance but then subtract about 0.12" for the extractor groove where the case is not supported and you are left with a minimum cylinder length of 1.06".
Maximum SAAMI cartridge over-all length for a .38 Special it is 1.55" and 1.59" for the .357 Magnum. Subtract 0.060" for the rim and then add 0.010" for clearance gives you 1.50" and 1.54" as the practical minimum for the length of the cylinder.
|The cylinder for a Ruger Blackhawk .357 Magnum weighs about 11.5 ounces.|
Shortening the cylinder from a length capable of supporting a .357 Magnum to just long enough to support the 9mm Luger immediately reduces the mass by about 30%. As noted before, you would also loose mass from the top-strap and bottom of the frame.
To be clear, most .38 Special and .357 cylinders are lighter than the one shown in the image.
Since the copper alloys suggested for the frame are 10% denser than steel and only as strong as steel that is not heat-treated, it might be necessary to increase thickness in selected areas to ensure durability. That might make it tough to meet a 20-to-24 ounce weight target.
To match the defensive potential of a Glock 19, the shooter needs a minimum of a full cylinder and two reloads.
To match the defensive potential of the Glock 19 and two additional, standard-capacity magazines requires a full cylinder and 8 reloads.
Reloading aids for revolvers can be awkward in shape. Anything that can be done to make them more compact or less obtrusive or lower volume is highly desirable.
There are some minor advantages to the 9mm. Since it is thermodynamically more efficient than a .38 Special it uses less powder to generate identical energy. Less powder means less muzzle blast.
Questions about the pressure differences are worth entertaining. While the .38 is half the pressure I am not sure that benefit can be fully reaped in material reduction in the cylinder because there are limits to how thin a section can be machined without the surface distorting. If the material is there for machining reasons then you might as well run the higher pressures.
The last thing I want to say about pressure in the 9mm is that it drops rapidly as the volume behind the bullet increases. My guess is that a Quick Loads comparison would show similar pressures in the barrel, so the only places where the structure would need to comprehend the pressure differences would be in the cylinder, and perhaps not even in the front of the cylinder.
So, given the ubiquitous nature of the 9mm Luger round, it seemed like a good candidate for the back-of-envelop study.