Carl Gustav m/42: A 20mm Recoilless Antitank Rifle

The Swedish Pansarvärnsgevär fm/42 made by the Carl Gustav company was an interesting early hybrid antitank weapon – a recoilless rifle firing solid armor-piercing projectiles. It used a 20x180mm case, propelling the 108g (1650gr) bullet at 950 m/s (3150 fps). This was capable of perforating 40mm of perpendicular armor plate at 100m (a high explosive projectile was also made). This was on the high end of armor penetration for anti-tank rifles, and the m/42 was able to do this with a weapon weighing just 11.7kg (25 lb) – less than a quarter of a comparable 20mm conventional rifle.

This was possible because of its recoilless design – upon firing, the rear end of the cartridge case would blow out and vent out the back of the weapon, instead of being firmly sealed like a conventional rifle. This created a counter balancing recoil impulse which prevented the gun and shooter from having to absorb the full recoil energy produced by a heavy bullet launching off at high velocity. The tradeoff was that much of the potential energy of firing was wasted venting out the back instead of pushing the bullet forward, which is why the cartridge case was so oversized.

About a thousand of the guns were made by the end of World War 2, at which time even it had been made quite thoroughly obsolete by the rapidly increasing thickness of tank armor. It would, however, be the stepping-stone to the m/48 Carl Gustav 84mm recoilless rifle, which used a shaped charge warhead to perforate armor with a stream of molten melt instead of relying on velocity of a hardened projectile.

35 Comments

  1. For absolute recoilless, the weight of bullet should be equal to the weight of powder charge. The cartridge of this rifle seems somewhat heavier at this ballance as if assisted by some construction creating venturi effect provided at the breechclosure side.

    • Technically, mass and velocity should balance in both directions. I.e. MV = mv, where capitals equal projectile values and lower case equal “countershot” values.

      The WW1 Davis Gun did it exactly that way, firing a shell forward and a ‘counterweight’ of small shot and axle grease (no, really) of equal mass backward at equal velocities.

      Most later recoilless guns work on the principle discovered in several countries in the 1930s, in that about .1 x mass of projectile in counterweight is fired aft at about 10x projectile velocity. As long as momentum/KE values come out the same, you get the recoilless effect you want.

      I’m not aware of any experiments of this sort going back to the American Civil War as Ian stated. But I have noticed that the British Armstrong and Whitworth breechloading cannon could have made entirely reasonable recoilless guns. All either one would have needed was a venturi attached to the breech on the Whitworth, or a set of two or four around the breech on the Armstrong, and a larger powder charge to achieve the mass/velocity balance.

      The Whitworth would have been especially suitable for this due to both its breech design;

      https://weaponsandwarfare.files.wordpress.com/2017/01/wsdxsdecw.jpg

      And the fact that its mechanically-fitted hexagonal bore required no “engraving” of a driving band to “take” the rifling. Most modern RRs have pre-engraved DBs on the shell and a ‘notch’ or something on the round’s case so it goes in the right way to make everything line up correctly. The Whitworth did this mote or less automatically. Its metallic cartridge case would have been very easily adapted to take a blowout disc in the base just as the M/42 has, as well.

      The main advantage of a Whitworth recoilless rifle over conventional artillery of its day would have been rate of fire in direct fire. Simply put, the crew wouldn’t have needed to manhandle the gun back into position after every shot.

      Also, Civil War gun crews already knew not to stand behind the thing when the lanyard was pulled, so the drill wouldn’t even have needed much changing.

      😉

      cheers

      eon

        • Counterweight principle was also used in experimental WW2-era German Sondergerät 104, 356-mm aircraft recoilless gun, see 5th image from top here:
          http://www.germanluftwaffe.com/archiv/Dokumente/ABC/s/SG%20Sondergeraete/Gereat%20104%20Muenchhausen/SG%20104%20Muenchhausen.html
          notice flash hider at counterweight’s end.
          This weapon was also codenamed Münchhausen named after Hieronymus Carl Friedrich von Münchhausen (better known as Baron Münchhausen), which according to query in English wikipedia:
          fictional Baron Munchausen is a braggart soldier, most strongly defined by his comically exaggerated boasts about his own adventures(…)the Baron rides a cannonball, travels to the Moon, is swallowed by a giant fish in the Mediterranean Sea, saves himself from drowning by pulling on his own hair, fights a forty-foot crocodile, enlists a wolf to pull his sleigh, and uses laurel tree branches to fix his horse when the animal is accidentally cut in two hence 1st image of top, of baron riding cannonball

          • The modern German Armbrust recoiless antitank weapon also uses the counterweight principle. Armbrust uses a container of plastic chips and is thus safe to fire from inside a room unlike a Carl Gustav or RPG-7.

        • I DOUBT that the Davis gun would pose much danger to the plane as the gun was mounted in such a manner to throw the counterweight ABOVE the wing, not INTO the wing! And besides, it was intended to attack U-boats. If I were hunting submarines (which can mount machine guns and artillery), would I use a biplane fighter with 2 rifle-caliber machine guns (cannot carry bombs without sacrificing range) or would I use a flying boat with a recoilless rifle pointed in such a manner that an attack run does not risk SMASHING THE FLYING BOAT INTO THE SEA? SAY SOMETHING!!!

          • That was the big advantage of the Davis Gun over both MGs and bombs. No need for a low level pass, and unlike a bomb it was pretty hard to miss when firing a fairly high-velocity gun pretty much straight down.

            During WW2 a similar ASW requirement resulted in the “retro-rocket bomb”, which when released fired a solid-fuel rocket in its nose to kill almost all forward velocity. It dropped almost straight down and was unusually accurate.

            Its most noteworthy use was by U.S. Navy Patrol Squadron Sixty Three (PatRon 63), the “Cowboys From Blitzville” aka the “MadCats”, based at Port Lyautey, Morocco. Their Catalinas, equipped with the retro-bombs and the (at the time still highly classified) Magnetic Anomaly Detector (MAD), made the waters around Gibraltar highly dangerous for German and Italian submarines.

            Fun fact; Photos taken of PatRon 63 “Cats” during the war were always carefully “posed” to conceal the MAD boom projecting from underneath the rudder.

            cheers

            eon

    • If the momentum of stuff going out the front of the gun is the same as that going out of the back, then it will be recoilless. Give or take transient effects when the bullet is still in the bore. By increasing the velocity out the back by lowering the exit cross section, the powder weight out the back doesn’t have to be as high, because the velocity would be higher. As the breech gets gas cut, recoil might go up because velocity of the gas goes down, but a heavier amount of gas might escape out the back.

      With an afterburner, you might have to go down range to retrieve the gun after firing. 🙂

      • I’ve read that the two British RRs, the WOMBAT (Weapon Of Magnesium, Battalion Anti-Tank) and MOBAT (MObile BAT), both jerked slightly forward when fired due to the thrust of the gases coming out their single venturis.

        There’s also a noticeable forward “pull” when you fire the Karl Gustaf 84mm off the monopod.

        cheers

        eon

        • The original BAT stood for Battalion Anti Tank. The blokes in the British Army who would have had to used it, reportedly, nick named it the VC gun; because destroying a Soviet tank with it would have resulted in Britain’s highest award for gallantry, probably posthumously.

    • It’s not a conventional antitank rifle, nor a shaped charge antitank weapon, but something in between without a good term to describe it. Sort of American slang meaning “I don’t know what you call it”.

      • “Sort of American slang meaning “I don’t know what you call it”.”
        I would say it is transitional that is having traits of old (AP shell) and new (recoilless system)

  2. Actually, shaped charges do work through kinetic energy. The high-velocity jet punches through armor much like water from a garden hose will burrow a hole into dirt.

      • No, for the most part it is not plasma. There is a very small amount of plasma generated by a shaped charge, but the mass is negligible compared to the total mass of the liner. The “jet” is actually not molten, either, but the extremely high pressure means that the whole penetration process is largely hydrodynamic (i.e. both the armor and the “jet” behave like liquids) in nature.

        Stickmaker’s analogy is quite good, except the armor actually behaves like liquid as well, so it’s roughly like a garden hose being directed at a pool. The smaller the diameter of the stream “jet” and higher the pressure, the longer the stream of water from the pool will go through the water in the pool (to see it you would have to use colored water of course).

        • Should of course be: Water from the hose will go through the water in the pool.

          Ian, can we have an edit feature to the comments section, please. These “ghetto edits” are a pretty crude way. They remind me of public men’s rooms…

  3. The reason this gun has felt recoil is the firing mechanism in the back of the nozzle. The 84 mm CG (which I have fired quite a lot) has the primer on the side, and nothing for the backblast to push against as it vents out the back. In fact, the weapon will dip slightly downwards when fired, as the balance of the piece changes with the detonation of the powder and forward movement of the grenade. The 84mm has a muzzle velocity of only some 250 m/second, with a booster pushing the HEAT round to 350m/sec after launch, slow enough for this dip to impact accuracy.

  4. There actually isn’t much difference between the muzzle energies and effective penetrations between the Carl Gustav m/42 and weapons firing the 20×138B long Solothurn cartridge. The latter included the Lahti, Solothurn S-18/1000 and S18-1100 AT rifles, various Finnish, German and Italian AA guns and their adaptations​ to vehicle use. Only the Germans had tungsten core APCR for their vehicles and it penetrated roughly the same as the Carl Gustav m/42 with its similar ammunition.

    Muzzle energy comparison:

    Carl Gustav m/42: APCR 108 grams @ 950 m/s; 48.8 kJ
    Lahti L-39: AP 152 grams @ 830 m/s; 52.5 kJ

    The muzzle velocity for the 20×138B projectiles varied a little depending on the weapon and of course projectile (MV for APCR was significantly higher, but I’m too lazy to dig up the numbers for the German APCR), but was always at least 750 m/s, typically around 800 m/s. The better penetrations of the CG m/42 is thus explained by the more advanced projectile compared to the Lahti and Solothurn ATRs.

    All this of course does not diminish the fact that the better portability of the Carl Gustav would have been a significant advantage in actual combat.

  5. Completely off topic, but every time I see an M1 clip loaded ‘wrong’ such that the top round is on the left it makes me cringe.

  6. So the “cartridge” holes have to be in-line with the holes of the recoil plate ?

    How was it possible to properly “orient” the cartridge ? Was there un “up” arrow ?

    • The plate appears to be the same fiberboard that the 84mm uses. Looking at the pictures of the 20mm cartridge it appears that the holes in the board are made when firing; the part not supporter by the recoil plate simply blows out. On the 84, the whole thing is blown of without a trace.

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