Wednesday, February 24, 2021

Why not .38 Special or .357 Magnum?

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

Size comparison: .38 Special, 9mm Luger, .357 Magnum. For all practical purposes the .38 Special and 9mm Luger are ballistically identical. From a packaging standpoint, the volume aft of the bullet in the .38 Special is 65% airspace since smokeless propellant is more energy intensive than black powder. Even though it is wasted space, the gun designer must still put structure (mass) around it.

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.


  1. Long as we're supposin'... Why not use a heavy +P .38 case and cut it down to 9mm length? Now you have a 9mm rimmed as it were so you don't have the hassles of moon clips.

    1. That works spiffy.

      However, it opens up the potential of some dummy loading a 35ksi .38 Sizzler into a .38 Special chamber. Past practice has been to make the rounds semi-idiot proof by making hotter rounds longer to prevent them from being chambered into less capable guns.

      In the "for what it is worth" category, the 9mm Luger case immediately in front of the extraction groove is nominally 0.010" greater in diameter than the .38 Special case.

  2. One advantage of the .38 Special is that, in a pinch, you can use homemade black powder. You would need a longer barrel, perhaps, but it would work if the modern .38 were not readily avialble due to politics or something.

  3. Thanks for answering my question. A WW2 Enfield mk II would be a good baseline comparison for what you're going for. It's a top break with a 5 in barrel and weighs in at 28 oz. It's chambered for the .38-200, which has an OAL of 1.240 so the dimensions should be close, but the max pressure for 14,500 is only 40% of 9mm. If you can balance removing unneeded steel or replacing it with lighter polymers with strengthening for the 9mm you might be able to break even and get around a 28 oz weight.

    Regarding reloads, have you considered cylinders with more than six rounds? I know you are going for minimum weight, but a rough calculation give me 8 rounds for a 16% increase in cylinder OD and 35% increase in cylinder mass. Area is a square function so the additional mass for additional shots slides downhill after that. The trade off seems marginal for current CCW use, but with mob violence becoming more common maybe its worth it.

    1. I think it is advantageous to go with an odd number of chambers because the cut-out for the cylinder stop is cut into the meaty part of the cylinder so seven shots would be on-the-table.

      There are some fabulous steel alloys and heat-treatments. One trade-off is that increasing strength can be impaired by poor impact strength, hydrogen embrittlement and so-on. A low-alloy steel like 4140 heat-treated to 30-to-35Rc can be expected to have a UTS in the 130-150ksi range and 20% elongation.

  4. I have a crazy idea for both the reloading and weight issue, single use disposable cylinders. The trick would be making them strong enough and cheap enough. I don't think 3d printing is there now but it probably will be in 5 years. My other thought was extrusions either aluminum or polymer. Tolerancing for cylinder gap would be a problem, but a Nagant style push forward cylinder lock would fix that.


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