MGD PM9 Rotary-Action Submachine Gun

The PM9 was an interesting an unique submachine gun designed by Louis Debuit for the French firm Merlin and Gerin (hence the MGD name – Merlin, Gerin, Debuit) in the late 1940s and early 50s. The design was intended to provide a very compact package, which it did with a very short action, folding stock, and folding magazine.

The PM9 uses a delayed blowback action, and the delaying is done by a rotating flywheel-type block and clock spring. The bolt and flywheel act somewhat like the piston and crank in an engine. As the bolt (piston) moved rearward in a straight line, it forces the flywheel (crank) to rotate because the two are connected. In the case of the PM9, the connection is a nub on the flywheel that rides in a vertical slot in the bolt. The flywheel is pushing against the clock spring to rotate, and the combination of the its inertia and spring pressure keep the bolt closed long enough for pressure to drop to a safe level. The rotary action allows this to be done in a much smaller package than typical submachine guns.

The PM9 was initially chambered for 7.65 French Long, but quickly changed to standard 9mm Parabellum for the production models. It used the same magazine as the German MP38/MP40, giving it a 32-round capacity. In addition to the model with a skeletonized folding stock, the PM9 was also available with a fixed wooden stock and either short barrel of long carbine barrel. A relatively small number of guns were produced in France in 1954 and 1955, but they failed to find commercial success. In 1956 the German Erma company acquired a license to build the PM9, but abandoned the idea after making a few prototypes.


  1. Thank you, Ian. I’d been aware of the PM9 for a long time and even seen a couple of diagrams, but I never knew exactly how it was supposed to work.

    Offhand, the only other weapon I can think of that uses a clockwork flat spring in a way that even remotely resembles this is the Lewis Gun, with its rack-and-pinion bolt carrier driven in counter-recoil by a clockwork spring in that round housing just in front of the trigger guard, after first being smacked backward by the gas piston assembly;

    The moribund H&K G11 also rotates its “breechblock”, but I believe that is done by the reciprocation of the barrel and magazine groups back and forth in the stock, rather than by a separate spring.



  2. Seeing as it can’t be fired with any practicality when folded, the recoil spring could have just been located in a tube in the stock, rather than the clock spring effort.

    • “recoil spring could have just been located in a tube in the stock”
      Spring in folding stock? Does anyone anywhere crafted working full-automatic weapon with magazine in spring?
      Such solution would need some device to prevent firing if not fully aligned, device to prevent attempt to fold when that spring is not in proper position, device for engaging and disengaging spring from rest, device to prevent dirt income when weapon is in storage mode.
      I doubt in possibility to make simple and reliable weapon with spring inside folding stock. I will consider it possible, as soon as anyone point me to working example of such construction.

      • Should you want to: You could have the recoil spring in two segments: the one that lives in the receiver and the other in the stock tube. Have the stock tube lock plate off both ends before folding and safe the firearm to prevent firing with only the receiver spring working.

        Alternately you could have the spring in the stock tube and connect it to the bolt with a rat tail with one or more ball and socket joints to allow the stock to fold.

        Not saying they are good ideas, but none of us are posting on ‘

      • I assume Richard meant to keep a protruding spring housing.
        But in that configuration, the overall length is longer even with folded stock. (or we need to shorten barrel).

  3. Actual rotating breech semi autos were developed. the BSA Ralock was a semi auto with a rotating bolt, that used a tube magazine, fired from an open bolt, and held the empties in a compartment, rather than scattering them around.

    There was a blogspot blog about open bolt semi autos, that had a lot of info on the Ralock, including copies of the patents.
    Unfortunately gargle is refusing to find it today, and refusing to play Ian’s video as well.

    • I tried a couple of different search engines and proxy servers.

      I found the blog, but all of the content has been taken down. The questions of “by whom?”, “Why?” And “under what pressure?” Remain.

      Here’s the address, if anyone wants to play with the way back machine.

      If anyone knows how to find historic caches with the posts up, I’d be very interested ​to know how, in order to use down em all or something like that to save a copy of all of the info on there.

      Censoring information is (IMO) tantamount to book burning – this doesn’t look good.

      • I was actively reading the blog when it got scrubbed. I clicked “older entries” and suddenly there were none. I wish I had the presence of mind at that point to archive important original research as I went along, but at least I know to do that now.

  4. Are there rotating bolt firearms where the bolt turns a full 360 upon firing, instead of reciprocating? (The only one I can think of is the H&K caseless subgun.)

  5. Merlin-Gerin, now part of Schneider Electric, is a maker of electrical appliances (switches, circuit breakers…) since the 1920’s.
    After WWII, they tried to diversify their product line. So they give a chance to small arms but this gets nowhere.
    If I remember correctly an article in the French Cibles magazine, they also designed other types of guns. If only I could find this single issue in the 300+ of my library!

    • I found the article! It was in Cibles #417 (December 2004) showing a wooden stocked version and a belt fed variant of this SMG and also a tripod mounted machine gun.
      But I found more: there were also a 2 parts article on MGD patents in Cibles #451 & #453 (October & December 2007) showing the first iteration of the flywheel action with a conventional spring in the buttstock and belt fed, blow forward pistols, machine guns, self loading rifles with grenade launching in mind and… caseless ammo!

      I can scan (and even translate) these articles if Ian want to post them on the site…

  6. Genius and madness are separated by a thin and permeable line oftentimes! I think it is brilliant. Look how compact the receiver is! And with the magazine folded forward and the stock folded, that is very compact. Just the thing for, well, I don’t know… bicycle riding police or clandestine operatives or something.

      • Ah yes, the Bonnot gang, what with the invention (I think) of the get away car. The illegalist anarchist bandits, erm “expropriators” at the time, Victor Serge included…Jules Joseph Bonnot and Pierre Jourdain… Brownings and a “little Bayard” vs. police revolvers and Lebel rifles…

        I guess you’re right about the motorcycles, insofar as the illegalists outran the bicycle in their horseless carriages.

      • “See The Bonnot Gang by Richard Parry;”
        For me that link doesn’t work? Anyone has similar problem?

    • “Genius and madness are separated by a thin and permeable line oftentimes! ”
      The distance between insanity and genius is measured only by success. (Tomorrow Never Dies)

    • “(…)And with the magazine folded forward and the stock folded, that is very compact. Just the thing for, well, I don’t know… bicycle riding police or clandestine operatives(…)”
      This reminded me about Burgess folding shotgun, which can also fit for such usage.

  7. I wonder if something like this could be adapted to the AR15 platform to replace the buffer tube and make for an even shorter folding stock version?

    I also am curious how that rotational motion would feel when firing. Would it cause the gun to have a rotational force in addition to the normal rearward recoil force?

    • Since the spring clockwork is probably balanced, I wouldn’t expect any net motion effect due to inertia, as there would probably be with an unbalanced “flywheel”.

      I suspect there might be a rather peculiar feeling vibration, though.



  8. What did it feel like when held in a firing position Ian? You don’t seem do it in the video. It actually looks like an awkward gun to hold, with very little space for the left thumb and the right forefinger; I guess you would not want to fire it left handed.

    Also: how many were made, and were any actually sold?

  9. Ian: You need to explain why it is “delayed blowback” rather than simply a plain blowback operating on a curved rather than linear movement of the breechblock.

  10. I see now that I was in error. The breechblock moves only in linear fashion.
    But my question remains: What, other than inertia and the recoil spring, causes “delay” before the breech can open?

    • Other than inertia, nothing — but the inertia of the spring and flywheel are linked to the breechblock by a varying ratio, much the same as in a toggle-delayed blowback.

      When the breech is closed, the flywheel’s pin is nearly horizontal, so the inertia is multiplied by a substantial mechanical advantage. (If the pin came all the way horizontal, of course, it would have infinite mechanical advantage, and it would be a locked breech rather than delayed blowback.)

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