The 'Stummelwerfer' and 2B25

3" Stokes mortar in use
Back during the First World War the Englishman Stokes invented the very simple Stokes mortar, using a smoothbore barrel, a fixed firing pin, a simple baseplate, a small sight and a tripod leg mechanism. It was a fascinating departure from the reign of rifled firearms that began three generations earlier and was a largely minimized design. The German tactical equivalent was a much more elaborate design, which used recoil buffering even though recoil could be transferred into the soil easily instead.

The French company Brandt refined the Stokes mortar during the 20's and created with its modèle 1927 a design that was the grandfather of all modern medium (81.4 and 82 mm) mortars. The further refined modèle 27/31 was introduced or copied in almost all countries. The similarities were such that any "8 cm", "81 mm" or "81.4 mm"* mortar could exchange ammunitions during the Second World War, and the Russian 82 mm was able to use these medium mortar munitions as well.

It should have been obvious that simplicity and light weight was a key to success with mortars, but plenty mortar designers fiddled around with unnecessarily elaborate designs. This included the German 5 cm light mortar, which disappointed as an impractical and weak effect weapon. It wasn't only the cost, bulk and weight of the weapon that disappointed the German army; otherwise, it would have made much more use of French 60 mm and Soviet 50 mm mortar models derived from the 81.4 mm Mle 27/31. The effect did apparently not justify the effort and 50-60 mm mortars were later mostly used by Germans in rear area security forces which received 3rd rate equipment typically.

kurzer 8 cm Granatwerfer 34
The German solution to the problem was to cut short the 8 cm GrW 34 (a Mle 27/31-derived mortar) and to limit the use of auxiliary charges therein. A truly crew-portable mortar was created, and it used the same ammunition as the regular ones. What I've seen in the literature so far indicates that this mortar was often deployed for direct fire.
This approach wasn't continued past-WW2.

The light mortar category was continued with "commando mortars" of 51-60 mm calibre (no bipod, tiny baseplate) following the lead of a Japanese WW2 design in philosophy and 60 mm mortars with bipod and real baseplate following Brandt's Mle 27/31 and Mle 35.

A development from the 1980's and 1990's was the introduction of long barrelled mortars in 60 and 81.4 mm calibres for extended range, but a much more intriguing and much more radical innovation was introduced for the "commando mortar" category in the 1970's*: The Belgian 51 mm Jet-Shot mortar. Its most famous designation nowadays is "FLY-K", but others such as "NR8113A1", "TN8111" and similar cryptic designations that are useless for marketing purposes were known as well.
The very special thing about it was its ammunition; it pushes itself out of the mortar using an internal combustion - but it doesn't vent he propellant gasses. This eliminates a lot a of problems, such as muzzle flash, most of the firing noise (replaced by a 'click' sound) and the mortar doesn't heat up any more (not very relevant with "commando mortars"). The drawbacks are substantial as well; more expensive ammunition, no ability to use auxiliary charges, limited range and no compatibility with other weapons.

FLY-K principle of operation

France and the UAE adopted the system, and France calls it "Lance-grenade individuel Mle F1".
There were copies of this innovation; a Georgian model was called Delta 60 mm silent mortar (by Jane's) and a Chinese model was known as Norinco Jerboa 51 mm, for example.

Finally, back in 2011 the Russians came up with such a system as well, and the pushed the range and effect boundary by using a 82 mm calibre with up to 1,200 m range (approx. double of the FLY-K's): 2B25 developed by Burevestnik (2nd link here). Its weight is more than double that of the FLY-K; 15 kg, which is still "man-portable".

This is interesting, as it's on the one hand a technically fascinating grandson of the FLY-K and grand-grandson of the "Knee mortar", on the other hand it's the first to follow the 8 cm krz GrW 42 at least a little bit; medium calibre and short range (company-level range). It does lack ammunition commonality with regular 82 mm mortars, though - albeit its silent 82 mm ammunition may be usable in regular 82 mm mortars. It would be plain wrong to make such a crew-portable short range mortar compatible with regular munitions (except maybe without any auxiliary charges) because this would require a stronger barrel; the "silent" ammunition doesn't stress the barrel much because it contains the overpressure itself!

A look at the technology may explain why the FLY-K was no world-wide success like the Stokes-Brandt mortar pattern: Restricted to low pressures and thus short range, no ammunition commonality, little effect of HE.
It's nevertheless technically fascinating and this may have driven its limited success and certainly kept alive the long-lasting interest. The FLY-K weapon has been adopted by a German company a few years ago; the journal articles about it shamelessly omitted that it's such an old design.
The Russians have corrected at least one deficiency - the light warhead - and this may lead to more commercial success. They may have semi-corrected the compatibility issue as well, since in theory it's possible to design silent 82 mm rounds for use in regular 82 mm mortars. I wasn't able to learn about whether they achieved this.

The Russians had several other "silent (captive piston etc.) designs earlier; including a silent underbarrel grenade launcher and very compact assassination pistols. They also invested a lot in 9 mm and 12.7 mm subsonic sniper rifles, which defeat the sonic boom-dependent acoustic sniper detection systems that became somewhat famous, miniaturized and mature in the Iraq and Afghanistan wars of occupation.

The "silent" 82 mm mortar (round) would defeat a multitude of means of detection as well:
(1) the hearing sense of hostile troops
(2) acoustic triangulation systems***
(3) flash-spotting by hostile troops
(4) flash-spotting by aerial or high vantage point sensors
All commando mortars have such a short range and low trajectory that detection by counter-mortar radars is most unlikely in warfare between armies.

Medium and heavy mortars may be in a survivability crisis, but the light and commando mortar categories sure have technically fascinating aspects. Their survivability concern is commercial; underbarrel and stand-alone 30-40 mm grenade launchers as well as electronics-assisted RPGs, Panzerfaust, bazooka and man-portable recoilless gun designs compete as satisfactory and often even superior substitutes. The "Armbrust" Panzerfaust even uses its own interpretation of a captive piston "silent" firing mechanism with no flash or smoke!****


*: All of these were actually 81.4 mm; only the Soviets differed slightly and the former Second World does till today, save for the new NATO members.
**: I wasn't able to track down its actual date of origin even though I paid attention to it for more than a decade, but it was no news back in 1985 any more.
***: Pioneered by the British artillery during the First World War.
****: With moderate commercial success, likely due to its poor weight-warhead size ratio. It has got much attention and keeps fascinating people since decades, just as the FLY-K. Guess which Belgian company purchased a production license for the Armbrust ...

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