H&K’s Experimental SMG and SMG II for the US Navy

In the 1980s, the US Navy requested a new submachine gun to replace the MP5 then in use. In particular, the Navy wanted a gun that was optimized for use with a suppressor. H&K built two models of experimental guns in the 1980s to meet this request, creatively named the SMG and SMG II.

Both guns were hammer fired, closed bolt designs that used simple blowback actions. They had collapsing stocks similar to what would later be used on the MP7, and vertical front grips. The early pattern had a remarkably complex rear sight offering two different sets of range calibrations for subsonic and supersonic ammunition (out to 150m). The SMG II reverted to a more standard HK drum rear sight, but added a very interesting velocity-reduction system. A pressure vessel under the barrel was connected to the barrel via a set of holes just in front of the chamber, and when opened it would reduce chamber pressure and thus velocity. It could reportedly reduce muzzle velocity from 350 m/s (1150 fps) to 305 m/s (1000 fps), thus bringing it below the speed of sound.

The Navy tested both models, and decided that neither warranted replacement of the MP5. No other sales were made of the designs, but much of the developmental work would be put into the UMP program that came afterward.


  1. As a student of weapons design and manufacture, I’ve always found it a bit frustrating that militaries bypassed the pistol-caliber locking / delaying actions already available in the interwar period for “brick in a tube” SMGs. As a student of the big historical picture, I of course recognize the value of mass-producing an MP-40 or PPS-43 during a desperate war of national survival (the five-Garand Thompson notwithstanding!)

    Now fast-forward to the 80s: with wartime pressure long past and the nation entering a post-stagflation boom, the best-funded unit in DoD asks the inventors of the world’s most highly advanced SMG for something better – and they offer 1918 derp-tech?! I’m sure the rejection positively shocked all involved.

    • Who pays can accept or reject whatever they want. Oh yes, technically speaking it is bit of kick into shin, but this is part of being in the business of military small arms – recoil, regroup and hit the market again.

      • Don’t get me wrong: I don’t begrudge any manufacturer (even one primarily associated with innovative and/or premium products) from trying to cover the whole spectrum of the market, even down to the entry level. When Ferrari was owned by FIAT-Chrysler, the fact that the same conglomerate turned out economy cars as well as world-class racers was just smart business – akin to HK’s later introduction of the UMP.

        OTOH, trying to sell this to the SEALs in response to the improved MP5 RFP was like trying to pass off a FIAT 500 (a carbureted one, at that) as Formula One.

    • What decrease in size or mass would “(…)the pistol-caliber locking / delaying actions(…)” provide for 9×19 mm Parabellum cartridge?

      • While I didn’t specifically mention decreased size or mass, it’s a valid question and a natural inference from the design principles involved – with many answers in the form of several SMGs and pistol-caliber carbines on the market today (the MP5 included). While its roller mechanism wasn’t ready to hand in the 1920s, the fundamental operating principle (inclined surfaces place a light breechblock at a mechanical disadvantage, opening slowly while accelerating a heavier rear bolt or carrier at 2-3x its velocity) was available in several other forms.

        While not a 9mm, the Reising is an illustrative example. It is a compact, <3kg .45ACP that Ian found very controllable – a compliment not earned by the same-caliber Thompson at twice the weight. The original Reising had some complexities and ergonomic shortfalls whose solutions are quite intuitive and could easily have been engineered out, had Ordnance recognized the inherent superiority of its mechanism and invested even a fraction of the funding lavished on upgrading the Thompson – ultimately resulting in an SMG that still cost more, weighed more, and climbed more than the Reising.

        For a direct 9mm comparison, take the Mauser Schnellfeuer, which Ian (and Patrick) also found very controllable with its stock attached. While it is striking today for its Victorian intricacy, Mauser managed not only to make it economically viable to sell to depressed, fragmented China, but even domestically producible there! It's clearly amenable to even further simplification (separate pins, separate barrel, coil springs, stamped lower). Optimize it for the select-fire role (beavertail grip, a heavier ~8" barrel, a barrel extension permitting an inch or so more free-recoil travel; perhaps a mechanical rate reducer if still found necessary after the other changes; plus a fixed or folding stock freed from the holster role and its bulk / complexity / excess LOP) – and you're left with a compact, controllable ~2.5kg select-fire carbine, completely viable for contemporary manufacturing processes, at a fraction of the R&D budget required to develop the bulky and harder-kicking Gen 1 SMGs from scratch.

      • Pistol caliber firearms need locking or delaying mechanism for;

        – Protecting both gun and user to be beaten through recoil and the recoiling parts violent impact at their returning travel,

        – Keeping the gun on the aimed location at follow up shots as soon as possible,

        – Providing highest supposed performance of the used round,

        – Extending gun’s usable life…

        Pistol caliber guns without locking or delaying devices keep working on for times longer than expected…

        But this experimental SMGs possibly contain a slowing device hidden in the back bumper similar to the one used in VP70 pistols which only slowing the backward stroke of bolt but not reflecting the collected energy to returning sequence. This kind of a construction provides most of the features expected from locking or delaying devices. What important at the matter, should be the cost of alternatives.

        Regarding to the muzzle speed decreaser, the effect of device should be positively computed at follow up shots… Gas is always quicker than mechanical devices and besides, a simple compression and decompession unit might be contained in there…


          • lf noticed, these “Closed bolt firing” SMG’s have small mass breechbolts, medium to weak level recoil springs with back bumpers movable by finger push… Seemingly the only slowing parts during the backward recoil are, the hammer and main spring which looking not sufficient for an affectable speed reducer for bolt rearward stroke… There should be a simple conical split friction ring or other inertial leverage hidden in the bumper for this purpose. lMHO…

          • Friction based devices are not reliable since they are subject to wear and their operating characteristic will change in time. Besides, for reliability’s sake it is desired to have clearance sufficient to absorb elements such as fine sand and not to seize.

          • Thanks Denny… A5 shotgun type split conical rings are obsolete… What l would like to point out, was a similar device contained therein… Like Belleville washers etc… HK should know what it was.

  2. The rear sight on the SMG looks like it could be a resigned version of the sights found on their belt fed machineguns(21/23 etc.).

  3. Later version has smaller mass of bolt with seemingly same lenght of barrel. Some change might be hidden inside the bumper piston at back as slowing the backward speed of bolt like the one VP70 pistol had.

    • You seem to be intrigued with bumper and I understand why.
      If it was just simply a plunger supported by spring it would not do enough of shock absorption. Maybe we should ask Ian to take screwdriver and look inside 🙂

  4. 1. The change from the goofy rear sight to the barrel-bleed device is interesting. Looks like the USN (can we say SEALs?) started out wanting to shoot 115grn and 147grn interchangeably, but someone later decided that 115grn at different velocities was a better way ahead.

    2. The sight mount, back then, would possibly have been used for early dot sights like the OEG, but was mostly there to accept NVDs.

    3. The “forward assist”, I suggest, is not a “forward assist” in the (misguided) sense used on the M16A1 etc. Rather, see PSG1, it is a quiet closing device. For SF/ninja scenarios where you need to chamber a round inconspicuously. Ride the charging handle forward, quietly, then equally quietly press the button to ensure the bolt is fully in battery.

    • Hmm… Silent closing should be needed for extra cool persons entering into a risky area without loading the chamber and getting the safety “on”…

  5. How did this warrant the expenditure of several million USD (at early 1980’s values) from which there came basically nothing.

    But even though the SEAL hierarchy knew through their liaison officer that the UK special forces had already been onto H&K from the mid 1970’s to develop what became the widely selling MP5SD series, one can ask but why then the expenditure?

    • The problem is called “government-trusted research.” Would you rather adopt a weapon on the basis of foreign hear-say or try to get your own custom item by direct order? Or, simply put, would the Pentagon resort to purchasing ALL of its service weapons on the private sector, with the weapons being little better than “civilian grade exports?”

    • “(…) why then the expenditure?(…)”
      Observing development of fire-arms for U.S. forces, I am more and more convinced about this:
      …the Soviet willingness to make the most of existing technology, as opposed to the American instinct of wanting to come up with the latest whizzy thing. http://www.airvectors.net/avsu17_2.html
      apply also to fire-arms and might explain asking for entirely new design, rather than examined fitness of existing ones to that task.

      • Or let’s sum it up: the idiots running the American procurement programs want the very best designs, if only to scare all potential opponents into giving up on waging war against America. Not that 1990’s Iraq was smart enough to not, uh, use rape/torture of civilian hostages/prisoners-of-war as a psychological weapon against a country that could easily reduce Baghdad to craters through aerial bombardment alone… just kidding.

  6. Genuinely ingenious device; what else would you expect of H&K? They are kings of the air and can prove it, again.

  7. Not exactly zee “kraut space magik” leserteknik von Hockler u. Kech, Geh Elf nicht war?
    On The Other Hand, this is definitive proof: “NOTHING is a better SMG that the MP5!”

  8. The cylinder on the SMG II looks like it’s good for a shot or two–and then the gun reverts to high velocity again. Gas takes up volume. Once the low-velocity cylinder is full, it needs to either vent the gas back through the barrel and suppressor or cool down. The effects operationally?

    The first shot will be quiet, and if there’s just one sentry dog or armed guard to eliminate, great.

    There might be a point-of-impact shift from Round One to Round Two (or whatever) when the low-velocity round is followed by a high-velocity round because the gas bleed cylinder fills up.

    Too bad that there’s no way to dig in the archives and find out what the effects are exactly. The G-36 rifle (also by H&K) exhibited accuracy problems due to construction that were undetected in service trials because nobody thought to run a dozen guns through an extended shooting trial to simulate infantry soldiers abusing the G-36 by putting out 600 rounds in under five minutes. That’s four magazines of 30 rounds per minute for the short duration of 300 seconds. That’s also three 200 round belts for the M249–the manual mandates a barrel change for that gun every 200 rounds, but with a cyclic rate of almost 900 rounds per minute a long 600 round belt will last something like 40 seconds of continuous fire. Yes, I know–do it “by the book!”

    So what would have happened if the SMG II had armed one of the Benghazi security team operators? Would he have stopped firing after one or two magazines and swapped weapons or would he have exceeded the recommended fire volume and launched multiple mag dumps?

    That system doesn’t seem to have been used elsewhere. If it were the greatest thing since sliced bread, I’d expect it to have been installed on many other firearms.

  9. That gun looks like a workable PDW, in terms of size and weight. With a folding hoodlum handle (or none at all) it could be carried hung from a belt hook, or even holstered, if you care to put up with that much plastic buckled onto your person. Most significantly, it’s the right caliber — at least, better than an economy-size .22 or .177. (You’ll pardon me if I have my doubts about sending men into danger armed with squib-firing weapons.) Dunno about the switcheroo gadget for changing velocities; how much call will there normally be for such complications?

  10. MP5 is not an army weapon. Too demanding on ammunition and moody.
    Like probably any delayed-blowback systems.
    In the device in question, an attempt was made to build a weapon with two shooting modes with one cartridge. No wonder nothing good happened.
    Just made it all the more complicated.
    More options for both technical problems and operator errors.

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