Julia: Shooting a 20mm Lahti L39

The Lahti L39 was the Finnish answer to the need for an anti-tank rifle, developed just before the Winter War. The rifle was created by noted Finnish designed Aimo Lahti, who had pressed for it to use a 13.2mm cartridge. However, arguments for using a 20x138B cartridge won out, based on hopes to use that cartridge in both antitank and antiaircraft roles, as well as testing that showed the 20mm projectile to have greater terminal effect.

The L39 was not available for use in the Winter War (having been adopted barely 2 months before the Russian attack), but was used extensively in the Continuation War. While improved tank armor quickly became thick enough to protect against the round, it was used for a variety of anti material roles, attacking machine gun positions, bunkers, light vehicles, and more. In 1944, an anti-aircraft version was also produced, firing in full auto and using 15-round magazines.

Today, ammunition is available from a few companies, typically using lathe-turned new cases and surplus 20mm Vulcan projectiles.

I also have an old interview video talking with Dolf Goldsmith about the few crimes actually committed with what would become regulated as Destructive Devices:



    • the peshmurga would use it, probably for defeating trucks or very light armor. you’re not going to use this on tanks made after the 1930’s.

      the PKK is not popular among the majority of kurds, as it is a communist organization. the turks use the PKK as an excuse to go after all kurds.

  2. While the nature of anti-tank guns limits their practicality for criminal operations, I can kinda understand the need to limit access to bazookas to some degree (even if it does mean a massive hassle for the enthusiast). Especially given the current climate. Also outside of a war-zone, the saying ‘The only thing that can stop a bad guy with a rocket-launcher is a good guy with a rocket-launcher’ is not really practical. (Also apologies if this may have dipped into the too political).

    • I wouldn’t dismiss this obsolete weapon, considering that it can still punch through brick walls and deliver explosive payload to any idiot thinking concealment equals cover. As for crimes committed with the L-39, those demonstrated how little lawyers understand technology and just how overblown the fear of old anti-armor weapons has become! You are more likely to get hit by a runaway train than get drilled with an increasingly rare auto cannon…

      • I’d agree. If mounted on one of those small all-terrain four-wheelers it would be formidable weapon system full of surprises (for the other side).

      • “I wouldn’t dismiss this obsolete weapon”
        Depend on conditions, if you have any automobile or motorcycle it should be possible to move it reasonable foot, but if moving by foot-slogging 50kg weapon looks very unattractive.

  3. If I might be allowed to be a bit of a film critic here …..

    An advertising logo works best when it seamlessly blends into the background. It’s probably much easier for indoor filming, where a logoed tablecloth or wall hanging can easily fit in with the decor of the room without looking too out of place. For outdoor shoots such as this one, I’d suggest using something more natural looking, or better yet, theme oriented, such as an old army surplus tent with the auctioneer’s logo painted on the side, or maybe even a common pickup truck as a backdrop, or whatever else that looks like it might actually belong in a grassy field.

    The way that the advertising banner was presented here, set up as a kind of makeshift draped clothesline on support stands in the middle of an empty field, just seemed very out-of-place and cheap. I’m not against advertising, I just prefer to see gently blended advertising, rather than something that slaps viewers in the face screaming. But if you’re stuck with it, I’d suggest zooming in on the tarp so it at least fills in the entire frame.

    Just my opinion, no offense intended.

  4. Nice and clear explanation and… actual action. Everything about that gun (and gunner) is real. It is kept in good shape. Well, enough for that.

  5. It’s interesting to me to note the placement of the front sight being so far back. One would think with all that extra barrel in front of it, sticking the sight further along would allow for a much greater sight radius for more precision at longer ranges.

      • Isn’t that also caused by human eye properties? I know that generally bigger distance between front and end better accuracy, but maybe is there some distance beyond which it don’t give further advantage? Or I am wrong?

        • For same angular movement with longer positioned sights you get quite a bit more “wiggle” off target; it takes more effort and concentration to align them. For close on weapons (used at no more than 300m) such as AK or vz58 it suits better if sights are closer together. It gets done quicker. Of course the error potential is bigger too.

          I do not think there is an ideal sights distance, especially now when iron sights are made obsolete with advanced optics. But if you look at past generation of rifles such as SIG550 or HK33, you get the idea. They have rear sight right at back of receiver.

        • The other thing which plays into it is size and shape of RS aperture. During my military training I took open notch rear sight as given; it was a norm for generations. The diopter type sight was for sharpshooters only.

          Later, when I came to contact with aperture/ ghost type rear sights I realised their advantage. They can be pretty quick at acquisition too and considerably more accurate.

  6. no quick follow up shots i guess, looking at the movement of rifle and shooter. why did they use this type of semi-auto action?

    • “why did they use this type of semi-auto action?”
      You ask about why gas-operated or something other?
      If first: it is possible that it was requirement of Finnish forces, but sadly I don’t know, if this is not case to get 100% accurate answer you would have probably have to ask Aimo Lahti (designer of this gun), sadly I don’t know Suomi – does he wrote memories?
      Obvious advantage of gas-operated is fixed barrel, which can give better accuracy and lighter weapon (than recoil-operated, where barrel have to have rigid enough guide).
      20x138B cartridge was used also in other weapons during WW2:
      Solothurn S18-1000: recoil-operated
      2-cm-KwK 30 (combat vehicle cannon): recoil-operated
      2-cm C/30 (naval weapon, Kriegsmarine): recoil-operated(?)
      2-cm-Flak 30 (AA cannon): recoil-operated(?)
      2-cm-Flak 38 (AA cannon, improved 2-cm-Flak 30): recoil-operated(?)
      2-cm-KwK 38 (combat vehicle cannon, improved 2-cm-KwK 30): recoil-operated
      2-cm C/38 (naval weapon, Kriegsmarine, improved C/30): recoil-operated(?)
      (note: above mentioned KwK, Flak and C/ weapons were associated)
      Breda 20/65 Mod. 1935 (AA cannon): gas-operated
      Scotti-Isotta-Fraschini 20/70 (naval weapon, Regia Marina): ?
      Lahti (AT and AA versions): gas-operated
      Nkm wz. 38 (in fact there exist few different design inside this development): recoil-operated or gas-operated

      • Solothurn S18-1100: select fire version of the S18-1000, installed on some Italian patrol vehicles in late 1942 and early 1943. Reportedly too inaccurate in full auto. Not used on ground mounts.

        Scotti-IF: first use was by the RM, but it was later used also by the Italian Army on an AA mount, which could be fired from the transport position, and was sometimes used like that against British light armor and soft-skinned vehicles in the desert.

        20 ItK/40 VKT: a twin barrel AA gun based on a beefed-up L-39 action, but not usually called “Lahti”. NOT the same as the unsuccessful L-39/44 full-auto version of the Lahti ATR. The L-39/44 rifles were all converted back to semi-auto only, because full auto fire would fracture the receiver pretty quickly. The 20mm M40 VKT AA gun, on the other hand, was very succesfull.

  7. In the U.K, some chap came up with a method to exploit a “loophole” in our restrictive firearms legislation, fairly recently. Lever release- Semi Auto rifles are banned, so we have straight pull ones (modern designs) to compensate… Anyway, his came up with a device that allows the rifle to fire then auto rebound I.e. The bolt, but it gets caught before the return stroke so to speak. Bit like this behemoth of a rifle, perhaps it’s based on that. Might not be, just saying.

    It’s pretty powerful this rifle, isn’t it. Rpgs are much less, ouch. But with decent clout, bit “artillery” this rifle, for a man portable firearm.

    • Scope for that kind of thing… To be developed. I watched that MAC channel, and he had a trigger that you could fit in an AR legally which fired I.e. Released the hammer when you let go of it. Thus speeding up the fire rate, to bump fire+ Speed.

      • “speeding up the fire rate”
        By how many %? Weapon’s RateOfFire might be increased, by various methods, but going beyond RateOfFire as calculated by designer might lead to many,many,many unwanted effects. If you examine development of aircraft-rifle-caliber-machine-gun of 1930s, in which RateOfFire was very precious, you will see that simply “speeding-up” existing design was possible but effects achieved were limited.

        • It definitely does, he has a video on it. You pull the trigger, semi auto AR, fires, back forward- Bolt. Then you release the trigger, fires again. Presumably that is quicker than pulling if again with your finger, see?

          • No, no… The weapon in the U.S is semi auto, so round chambered, pull trigger, hammer falls, bang, bolt moves rearwards, re-cocks hammer, now… When he releases the trigger, it releases the hammer? Thus, it fires again. Instead of, release the trigger and pull again.

          • The ATF could easily slap down such a dual-cycle trigger as an illegal machinegun, based on the fact that the release-stroke requires a spring to fire. Although there is absolutely nothing in Federal Law that regulates springs in any way on a firearm, we saw how the ATF created out of thin air a new rule regarding bump-fire stocks on a semi-auto rifle: without spring = legal rifle; plus spring = illegal machinegun.

            So the NFA’s actual written definition of a machine gun stating “one continuous pull of the trigger” is completely meaningless, since slamfire and spring-assisted bumpfire and God-knows-whatever-else can simply be added by the ATF to the legal definition of ‘machine gun’ as they see fit.

            I’ll make a prediction here: the ATF will reverse its opinion of the legality of dual-cycle triggers, and thereafter only allow them if they are human-powered in both directions (i.e., no trigger return spring).

          • Why does the ATF not just say “ALL possible means of simulating automatic fire, whether by springs, lack of control mechanisms, user grip technique, or by improvised frame-works shall categorize any subjected firearm as a machinegun” and then subject the “offender” to summary execution by conveniently timed car accident?

          • That was an unpaid, but otherwise favorable endorsement.

            “Think you have to say that in the U.S, got it from Guns and ammo, boots and such ads”

        • Oh, I’m not getting technical… I mean, fun. You know, for fun. Bbbbbbang. If someone owns an AR and wants to impinge on it’s molecular make up, that is fine by me.

          • Warp drive? Never mind, he he.

            I mean, your impying said device would probably be bad for the weapon. I defer to your superior knowledge.

    • ““loophole””
      How is self-loading rifle defined. Would be rifle self-loaded it would always hold-open bolt, which could be later disengaged by trigger pull i.e.
      1.trigger pull = shot fired, bolt opened
      2.trigger pull = round chambered, bolt closed
      3.go to 1.
      I.e. it would fire each two squeezes of trigger

  8. MAC = military arms channel. A good site. Google it and you will find the videos on the trigger Pdb talks about. It is a semi auto trigger pack where you fire a shot when pressing the trigger AND fire a shot when you release the trigger after you have pulled it for a shot. Very legal according to the BATF and very fast (in the range of 700 shots a minute?)

    My question about the lahti is why did they use the semi auto system where you have to release the bolt after each shot and not a true semi auto? Simpler to build? Stronger?

    • “My question about the lahti is why did they use the semi auto system where you have to release the bolt after each shot and not a true semi auto? Simpler to build? Stronger?”
      Справочник по стрелковому оружию иностранных армий states that it provide more intensive barrel cooling

    • Possibly they catch the bolt and carrier for recoil reduction. You can search for momentum transfer in inelastic collisions. Basically the bolt has momentum -mv which is transferred to the rest of the rifle. After the collision the bolt has zero momentum relative to the rest of the rifle. If allowed to slam off the back and continue back to the front it has momentum mv. An inelastic collision transfers mv momentum while an elastic collision transfers 2mv.

    • In Northern Europe there are many users of this shape of flag, but differing by colors, not mentioned yet:
      Sweden: yellow cross, blue background
      Norway: blue cross, white border, red background
      Denmark: white cross, red background
      Føroyar: red cross, blue border, white background
      Iceland: red cross, white border, blue background
      Notice that lands (or its parts) of above mentioned entities were once in Kalmarunionen: https://en.wikipedia.org/wiki/Kalmar_Union
      which also had flag of this shape (red cross, yellow background)

      • Åland Islands: red cross, yellow border, blue background (Åland is an autonomous demilitarized region of Finland)

  9. A Finnish John Browning…

    Unlike the Mormon JMB, the ?Lutheran Lahti liked an occasional drink…

    or ten.

    We’ll never know what wonders Lahti might have come up with, if his career hadn’t been cut short by post war restrictions on his design work.

    use of 20mm incendiary rounds to set forest and heathland fires, was supposed to be partly in the hope of illuminating the soviet forces from behind.

    It was also claimed that the 20mm could be used to break track pins, and to stake the turrets and guns of later tanks from moving, even if it couldn’t fully penetrate the armour. Once immobilized and with the turret and gun staked in place by impact craters, other means could be used to capture the tank, for repair and re-use.

    With its peat bogs, lakes, forests, boulders (and its c 4 million inhabitants) Finland was definitely not a good place to try to use tanks. The Soviets even found their own two best generals fighting for the Finns,

    Generals Janvier et fevrier

    • “The Soviets even found their own two best generals fighting for the Finns,”
      Soviets also underestimated Finnish forces, lacked winter equipment and commanding skills of Voroshilov were (at best) dubious. Nonetheless due to advantage in soldiers and weapon Finland eventually surrendered, Red Army use experience gained.

    • “We’ll never know what wonders Lahti might have come up with, if his career hadn’t been cut short by post war restrictions on his design work.”
      Officially he was banned from weapon designed due to 30 missing assault rifle, but I don’t know whatever it is true or hoax.
      Anyway, according to http://www.jaegerplatoon.net/ALMOST1.htm
      Lahti designed L-34 Sampo gas-operated light machine gun which was simpler than Lahti-Saloranta M/26 (58 parts of 118 parts) and proved be reliable in test, but was not produced – Finnish ordnance was prejudiced against gas-operated weapons and Lahti-Saloranta M/26 was already in production.

      Sampo is artifact from Finnish mythology, which brought good fortune to its holder and/or mill that made endless goods from nothing.

    • The notion that Finland is, or even was back in the 1940s, all bad tank country is partially a myth, albeit one still believed by many Finns as well, thanks to certain infantry traditions and misconceptions going back to the Winter War.

      The Karelian Isthmus, which was the Red Army’s main attack route to Finland in 1939 and again in 1944, was also good agricultural land and had a reasonably large areas of open fields. Not wide open and flat or gently rolling like much of Poland or Ukraine, of course, but not just forests and bogs, either. In 1944 Soviet tanks were often able to utilize the more open areas quite well. Finnish defenses had to be designed around natural defensive positions, where infantry anti-tank weapons could be used effectively and towed AT guns could be well concealed. Failure to do so almost always lead to Soviet tactical advantage with often bad consequences for the defender, unless the limited Finnish armor could be brought to bear for a counter-attack.

    • Which is why old anti-tank rifles are impractical as weapons in crime: TOO FREAKISHLY EXPENSIVE!!!

      Same goes for large caliber artillery…

      • I’d be willing to bet that when these were first imported, surplus ammo was dirt cheap, and maybe that was a big reason why these guns seem to be much more numerous in the USA than any other big anti-tank rifle.

        Also, these big guns appear to be more legal in California than a .50 BMG, or at least new registrations are not specifically banned by law, but just requires a Cali state permit in addition to ATF permit. Actually getting approved is of course another issue.

        • Before the 68 gun control act they were pretty common. I talked to a guy who owned one back in the day. He talked about it not being too expensive to shoot and how much fun it was to fire it into spoil banks at the strip mines near where I used to live. I wish we could bring those times back today.

  10. Ian-

    In the slow motion it looked like this rifle pushed you back quite a bit, especially considering it is 50kg and you’re firing prone with a staked bipod. On the scale of guns with tremendous recoil, did this knock you back more or kick harder than that 4-bore you shot a few months back?

  11. Is it the end of the work day? or lunch time? when you are firing machineguns, African big bores, anti tank guns etc. in the parking lot? or is that just a ‘hazard’ of the work day at James Julia?

    Also did after the initial posting of the Dolf Goldsmith video did anyone look up the bank robbery case in question? wiki isn’t much help.

  12. As always, it was terrific to hear Dolf Goldsmith share his experiences — absolutely priceless, to put it mildly.

    The bank robbery he mentioned in which a 20mm Lahti cannon was used to blast open the door to the vault became, I believe, the probable basis of the 1974 movie “Thunderbolt And Lightfoot”, starring Clint Eastwood and Jeff Bridges as the two partners in crime. The rest of the cast was quite impressive too — George Kennedy, Gary Busey and Geoffrey Lewis. The film was written and directed by Michael Cimino and produced by Robert Daley.

    The part that I remember most clearly from the time I watched it 42 years ago was the scene where they set up the cannon and shot out the locking mechanism on the armored vault door at the “Montana Armored Depository”. The movie is still available from various website order houses, Amazon.com being one of them. The standard DVD is still reasonably priced ( US$11.53 new on Amazon ), and the Blu-Ray version is available for as little as US$8.46 ( again new from Amazon, but you will probably have to have a multi-regional Blu-Ray player since this is an import ) or as much as US$38.89 for a new US/Canadian compatible item.

    • We had a similar robbery here when I was in high school. in fact it was at my high school, which had a medium-security vault room in the office area. The target was several thousand $$$ collected in a fund drive, which the burglars walked away with.

      Except that instead of blasting their way in through the door with a 200mm Oerlikon or whatever, the burglars rather more sensibly went in through the back of the vault (which faced a corridor)with a burning bar;


      To avoid the problem of incinerating the contents of the vault, they chose a section of the back wall that had nothing in front of it, cut a 2′ hole through the cinderblock, slung an asbestos mat over the bottom half of it, and then one shinnied inside, deactivated the alarm and time lock, and then opened the door from the inside with the emergency release.

      Now that’s sneaky.



    • You are correct. I should have mentioned that too, but I was only saying that the real-life robbery could have inspired the making of the movie ( with the usual artistic licence, of course ).

  13. I can rememberthe adds in guns and ammo for this gun befor 1963
    99 dollars for the gun and ammo was a dollar a round
    In the same add deactivated inngram mk 6s were about 25 dollars

  14. It would be nice to get instruction manual on Lahti L39,is anyone have it???
    It interesting to see how cranking cocking handle operate in detailed diagram..

  15. cant help but think that Syracuse bank job the inspiration for the Clint Eastwood movie Thunderbolt
    and Lightfoot inwhich he did the e thing with an Oerlikon 20 mm Cannon.

  16. My grandfather was actually trained to operate these during the Interim Peace aka in 1941. They were training in Kemi (small coastal city at the North-West Finland) during the winter time and shooting at the small island. After every shot a man and the gun slided several meters backwards. Many broke their bones because they weren’t holding the gun tight enough.

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