M20 75mm Recoilless Rifle: When the Bazooka Just Won’t Cut It

The M20 75mm Recoilless Rifle was developed starting in 1944 as a replacement for the 3.5” bazooka in an antitank role. It was developed and produced in parallel with a 57mm recoilless rifle (the M18), and both entered service in March of 1945, seeing just a slight bit of combat use before the end of World War Two. It would be a mainstay of US troops in the Korean War, however, along with a 105mm recoilless rifle. The M20 fired HE, HEAT, and WP (smoke) rounds, with the projectiles weighing 20-22 pounds (about 10kg) and having muzzle velocities of about 1000 fps (305 m/s). The shaped charge HEAT warhead could penetrate about 4” (100mm) of armor, and had an effective range of about 400 yards. The HE warhead could be effectively used out to about 1000 yards, and the gun was equipped with both direct fire and indirect fire optical sights in order to effectively use both types of ammunition.

By the Vietnam War, the M20 was on its way out, as were recoilless rifles in general – they were being replaced with wire-guided missiles for antitank use. However, the M20 remains in service today for avalanche control in many Western states – a neat repurposing of obsolete weaponry!

46 Comments

  1. Please clarify – was it one optic that could be mounted in two different position, or two different optics?

  2. In the mid sixties while serving in the army as a small arms repairman I spent two years in the 4th Armor division. The infantry had quite a few 105mm’s. We had very few problems with the main gun but a number of problems with the 50 caliber spotter rifle which was mounted paralell to the main tube. It had an interesting trigger mechanism. There was a knob facing the shooter inside of the traversing and elevating mechanism. If you pulled the knob it fired the spotter and when the white phosphorous bullet hit the target you simply pushed the knob and the main round followed. The 50 caliber round was a shortened version of the 50 BMG. The tragectory was identical to the 105.

  3. One minor quibble.

    The “Super Bazooka” (M20 3.5in rocket launcher) was never issued during WW2. It was developed in 1944-45 precisely to deal with the heavy German Tiger I and Tiger II tanks, but the war was over before it was ready for production.

    It lay unused for five years, then when the T-34 became a problem in Korea, development was finished and production began for use there.

    https://en.wikipedia.org/wiki/Bazooka#Rocket_Launcher,_M20_%22Super_Bazooka%22

    Incidentally, the 57mm, 75mm, and 105mm recoilless rifles were known as the “Kromuskit” type, after the names of their designers at the Infantry School, Kroger and Musser, who developed them “off budget” while Army Ordnance was busy trying to copy German recoilless guns with blowout-base cartridges;

    https://en.wikipedia.org/wiki/M18_recoilless_rifle

    The Kromuskit type works more like the British Burney type, except with more numerous and smaller vents in the cartridge case and overall simpler breech design.

    cheers

    eon

    • The Burnley design had the very real advantage of having the Venturi throat built into the base of the cartridge rather than into the breach. Hot gas erosion of the Venturi throat iisnt an issue as it is on the US guns. I’d also argue that the falling wedge breach of the Burney guns is an easier design to use as its a single action rather than the two needed to open a Kromuskit gun.

      • Cursed auto correct! Burney not Burnley . . . and I’m now wondering if the BAT, MOBAT, WOMBAT series were Burney besigns anyway.

        Best to shut up before I totally prove my idiocy!

  4. Avalanches are not to be taken lightly as they can happen without warning. No wonder this type of artillery is needed for that civilian purpose, because the alternatives are probably too dangerous (planting explosives by hand or having planes drop bombs can get a bit dramatic). I could be wrong about this.

  5. One of aspects I like on FW is that it does not limit itself to held-held arms. This is and excellent example and Ian’s innate ability to explain things in easy to understand fashion, multiplying the informative effect. As usual, good job and thanks.

  6. There is often mention of “easy detectability” with weapon of this kind. What comes to my mind is what was meant as a precaution; frequent replacement into new position? That is perhaps possible with relatively light weight. I suppose training was along that line too.

    But, is it not same as with ATGMs which are in frequent use today? Maybe it is not that bad. They leave some amount of smoke on take-off, more or less. But with them it is mostly one off and that’s it, job done.

    • When a recoilless rifle fires, there’s a literal blast of flame out the back, distinctly larger and more noticeable than most modern ATGWs. The closest comparison I can think of is the backblast from a single round from a WW2 German Nebelwerfer MRLS. As Willy Ley said (Rockets, Missiles and Space Travel, Viking Press 1954), that blast was brilliantly luminous at night and even in broad daylight kicked up a large dust cloud.

      For real “havoc before and behind”, look up the British 7.2in Burney recoilless. Four vents, and every firing looked more like the gun blew up.

      Incidentally, this sort of thing made the use of recoilless guns in urban operations a bit problematic. Even more so than a Bazooka, Panzerfaust or Panzerschreck, you had to be a bit careful where you sited one to avoid the backblast being directed into a confined space also occupied by the gun crew.

      cheers

      eon

  7. Replaced the .35-in M20 Super Bazooka? I think not:

    https://www.globalsecurity.org/military/systems/ground/m20-bazooka.htm

    Besides, different tactical niche. History gets complex. There were recoilless rifles in 57mm, 75mm, 90mm, 105mm and 106mm in US service–and the Davy Crockett, too! The Ontos anti-tank system mounted six 106mm recoilless rifles and a pair of .50 caliber spotting guns on a light tracked “armored” vehicle and the Marines used them, but never for their intended purpose as a tank killer. Today, SpecOps Command uses the 84mm Carl Gustav for those instances where guided missiles are not the appropriate tool. Then there’s the SMAW in Marine Corps rifle companies–the old B-300 repurposed.

    During World war Two most bazooka rockets fired by American infantry were expended on enemy bunkers, machine gun nests, and other stationary targets–and reports were that this portable Sunday punch was much appreciated because it was right up front with the rifle platoons.

    The Super Bazooka was replaced by a number of different systems (along with the anti-tank rifle grenade) starting with the M-72 series Light Anti-tank Weapon and the Dragon anti-tank guided missile. The Dragon was alleged to be man-portable and was issued down to squad level in USAREUR until the end of the Eighties.

  8. I recall photos of Marines using these in Huế during the Tet offensive. And the Marines had the M-50 Ontos in Vietnam as well.

  9. I read the Marine Corps manual on 106mm RR’s in the late 60’s. It stated the barrels and breech would get so hot after 5-6 firings that the combustible ammunition could cook off after loading. Another reason it didn’t stick around?

    • A friend of mine crewed on an Ontos at Huế. He said that after about three six-round volleys, they had to leave the breech blocks open and preferably point the guns into the wind if there was any to get airflow through the barrels to cool the chambers. Otherwise there would likely be at least one or two “premature firings”, which is not a good thing with an artillery piece.

      cheers

      eon

  10. The 75mm recoilless rifle was used in our unit against the Chinese at ranges of 3000 yards and more. It was effective against dug in the side of mountains Russian 75mm rifles. Nasty recoil. All troops who were tasked with firing them are deaf now. Our hearing aids are mute witness to this slight problem with this gun.

    • A very good point. I am sure I remember reading once that the Germans ordered recoiless rifle crews in WWII to use ear plugs for this very reason, as the blast from these guns is immense. Ear plugs are hardly novel or high tech, I wonder why the US Army was so cavalier with the welfare of its troops?

  11. Mounted backward on the tripod as it is, it looks like that’s about all the elevation you’re gonna get out of it. To get maximum range for indirect fire you’d need to be able to elevate it to near 45 degrees. The tripod setup would probably be best employed on a hilltop or tall building where having a greater range of depression would be advantageous.

  12. Err… as far as I know both of the early American recoilless guns (57mm M18, 75mm M20) were not intended as primarily anti-tank weapons, nor as a bazooka replacement. Nor were their effective range in direct fire as short as 1000 yards.

    Yes, the later 90mm M67 and the 106mm M40 weapons were intended as primarily anti-tank weapons and yes the 90mm M67 recoilless gun did replace the 3.5 inch rocket launcher (bazooka). But the WWII era weapons were intended for the role of infantry guns, like the Japanese Type 92 battalion gun or the German 7.5cm IG 37. And as John Wallace wrote they were used very effectively against enemy fortifications at long range. Very well emplaced Chicom fortifications up in the sides of Korean mountains which only the hand carried recoilless guns could effectively attack.

    The Chinese even copied and mass produced the 57mm M18 and 75mm M20, which saw combat use in the hands of VC and NVA forces against the Americans and ARVN during the Vietnam War.

    In fact even today, recoilless guns fit such an important role in infantry combat that in their absence the impressment of anti-tank guided missiles are used as a very expensive substitute!

    • Technically speaking, quite a few modern ATRLs (anti-tank rocket launchers) are more recoilless guns than anything else.

      The ubiquitous RPG-7 family uses a recoilless-type charge to launch the round from the tube, followed by the actual rocket motor firing when its 15-20 meters downrange. This allows for a longer motor burn and thus greater range without endangering the operator with rocket backblast.

      The Bazooka and its kin, of course, used rocket motors that literally were “all burnt” before leaving the tube for the same reason, which severely limited their effective range on a moving target.

      Against a bunker or etc., it didn’t much matter, as the tube could be elevated to launch the round rather like a rifle grenade or mortar, therefore its relatively low velocity wasn’t a problem. The target wasn’t going to be dodging before it got there.

      cheers

      eon

    • That I can easy understand – ATGMs have fancy sensors/ guidance units which are likely not cheap affair. In final count is sounds like any ‘economy’ calculation: loss vs gain. Tank costs more than missile fired against it.

  13. Problem with M20 and bazookas in Korea stems more from HEAT shells in use vs. any flaw of launchers themselves.

    Early HEAT warhead designs were highly variable in performance vs. fuse design, often the fuse would not function esp when hitting sloped target e.g T-34(this in some designs took until 1970s to fully resolve). In other cases combination of short ballistic cap and liner was such the warhead often impacted target before fuse mechanism could properly detonate the main charge, resulting in crush up. To make matter worse in above case, crush up produced effect resembling HESH round partly masking the malfunctioning fuse. see below for detail

    http://www.dtic.mil/dtic/tr/fulltext/u2/a220095.pdf

    “In 1951, this writer was invited to observe infantry training at Camp Roberts, California, where it was obvious that the 2.36-inch Bazookas were, for the most part, failing to detonate high order and form a jet as designed. Instead, most of the rounds were apparently functioned low order from crush-up on the target, as evidenced by the presence of many undeformed conical liners laying about on the test field. Further, the damage to the armor targets usually resembled that produced by a HEP or squash head mechanism. Even the Army instructors seemed to be unaware that their Bazookas
    were malfunctioning. They described the Bazooka’s terminal effect as “discharging a baseball sized chunk of metal from the far side of the armor.” There was no mention of a penetration hole.”

    Task force Smith in their disastrous encounter with NK T-34 almost certainly experienced the above, with possible additional complication from deterioration of munitions stored/maintained in indifferent conditions following the draw down post WWII (M1 carbine failure in same time period likely has similar origins).

    P.S Both M20 and super bazooka were copied by PRC and used against US forces during Korean war; in some cases the former was main infantry HE weapon used against US forward entrancement in the battles for ridges. Both later ended up as mil aid to north Vietnam following Chinese standardized on Sov pattern weapons later in 1950s.

  14. Speaking of tank-killing artillery, I recall that there is one really humiliating way for the M4 Sherman to meet a possible demise: get shot point-blank in the flank hull armor by a Type 4 Ho-Ro’s 15 cm howitzer (or by any self-propelled artillery not intended for direct fire). I know it’s unrealistic, but I met such a nasty end in online video game form: I forgot to check the flank when participating in a “realistic” level tank fight (no helpful “hostile team markers” to tell me where the other team was hiding) and charged right past a Type 4. The player controlling the Ho-Ro had disguised it as a bush (with premium camouflage kit) and let me have the HE shell right in the ammunition rack. You can guess what happened to the M4A1 I was driving. Similarly, it is also recommended not to charge at a Sturmpanzer II that has yet to fire its main gun, even if you are driving a cruiser tank. I could be wrong…

    • “(…)15 cm howitzer(…)”
      Regarding of Great Patriotic War-era: even if artillery piece designed as field guns is able to penetrate enemy tank armour, it does not mean it will be effective in that role. Main question is how much barrel can swing to left/to right without moving whole gun, how fast it can be done (train rate) and if it has sights suited for that task.
      Nonetheless if it hits…

    • “Sturmpanzer II”
      This vehicle was armed with modified 15 cm sIG 33, thus it could use HEAT shell penetrating ~ 160 mm.

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