Exotic Barrels Part 1: Squeeze Bores

In 99 repeating 9% of gun barrels, the caliber is what it is, and the bullet that comes out of the barrel is the same diameter it always was, just marked by the rifling. Likewise, the rifling twist is what it is, and from the point where is picks up in the leade (forward of the chamber) to the point where the bullet exits the barrel it is constant.

Then, there are the exotics, the ones that keep 99.9% from closing the gap between there and “all.” We’re going to talk about one exotic bore, and one exotic twist, in a pair of posts: Squeeze Bore and Gain Twist. Even though the names sound dreadfully like 1970s NATO codenames for Russian anti-aircraft radars, they’re both really a thing.

Squeeze Bore

The idea behind squeeze bore is to use the power of the powder to forge the projectile down in diameter. This would, in theory, do one of two things: blow the gun to Kingdom Come, or accelerate the projectile to velocities previously unheard of. It didn’t take long for people to try to reduce this theory to practice. The 1957 edition of Naval Ordnance and Gunnery, Volume 1:  Naval Ordnance, a training manual coded NavPers 10797-A, showed five different ways to get high velocities. The first is the familiar expedient of a lighter projectile, and the second, the saboted projectile used in most tank KE rounds these days, and in the .50 SLAP (saboted light armor penetrator) round. The third example, essentially beefing the gun up to take excessive pressures, doesn’t seem very practical, and the fifth was, in 1957, science-fiction stuff but is now a pretty routine way to get longer ranges in artillery. Which leaves the fourth example, D, our squeeze-bore

CHAPTER-6-E-FIGURE6E1-PAGE-95

A very, very gradual and subtle version of squeeze bore is the choke used on some firearms. But there’s nothing subtle about true squeeze bore. The World War II German Pak 41 fired a Gerlich-designed 42mm projectile, which the barrel squeezed down to 30mm at the muzzle. At around the same time, the US developed (at Frankford Arsenal) squeeze bore M2HB barrels, which fired a special bullet that squeezed down from .50 to .30 caliber. These guns produced extremely high velocities, with kinetic energy and penetration to match.

S.PzB.41 in action (or at least, being demonstrated). Wheles were removable to lower silhouette.

S.PzB.41 in action (or at least, being demonstrated). Wheels were removable to lower silhouette. Troops show scale… this is really small for something that can ding a JS-1’s frontal armor.

Squeeze bore was primarily used experimentally in antitank weaponry. The one weapon fielded with a squeeze bore was the German Gerlich S.PzB 41. The name Schwere Panzerbüchse meant, literally, heavy anti-tank rifle, and the Germans may have seen it as a replacement for the 7.92 x 94mm PzB 39, but its lightest variant weighed around 300 lbs. It could be broken down into smaller, man-portable-for-a-short-distance, loads.

Factory photo of the stripped-down paratroop version.

Factory photo of the stripped-down paratroop version.

The effect can be approximated by firing an oversize cartridge in a smaller-caliber bore, if the throat or leade is not too tight. (If it is, you get a kB! instead). You’re more likely to get away with such an inadvertent bore squeeze if the projectile is highly malleable, like a soft lead bullet. The Gerlich system used a tungsten penetrator with an aluminum alloy jacket, including crushable skirts. The projectiles looked like this (HE/frag on the left, with a filler of phlegmatized PETN;  AP with a tungsten-carbide penetrator on the right):

28-20 squeeze bore

(source)

The S. PzB. 41 was very effective; at close range it could penetrate all mainstream Allied armor (even the KV-1 and JS-1 tanks), although its behind-armor effect was limited. The Germans were successful in making squeeze bores where other nations’ designers had failed. They mounted it on SdKfz.250 half-tracks and used it as a trailered, man-packed and airborne weapon.

A larger squeeze-bore, the Pak 41, was deployed in small numbers. The ammunition closely resembles the 28/20mm of the S.PzB.41 but is much larger: it started off at 75mm and squeezed down to 55mm. An intermediate sized version was a 4.2 cm (42mm tapering to 28mm) squeeze-bore version of the familiar Krupp 3.7cm light anti-tank gun. (German guns are described in centimeters — move the decimal point once for mm — and their squeeze-bores are known by their initial, not squeezed, caliber).

Pak 41 APBCT

Making a tapered or “squeezing” rifled bore is a challenge, if you think about it, and conventional methods of rifling such as buttons and broaches don’t adapt well to it. (Cut rifling does adapt, but at a price in complexity. But the German invention of hammer-forging barrels over a mandrel opened up mass production to squeeze bore in German plants. (A microscopic amount of taper is usually used in hammer forging, to facilitate mandrel removal. But the amount of taper in a squeeze bore is much greater).

The British made a theoretically sound and plausible attempt to work around the difficulty of drilling and rifling squeeze bores. This was a squeeze-bore muzzle attachment called the Littlejohn for the 2-pdr antitank and light-tank gun, in order to give some realistic anti-tank capability to the airborne (glider-delivered) Tetrarch light tank and various wheeled AFVs.

Littlejohn_Adaptor_Bovington

It squeezed the round after it had been spun to speed; the holes you can see were for pressure release. The Littlejohn was conceived by a Czech emigré, Frantisek Janacek (whose name means “little John”, literally) and was made for the 40mm Vickers S gun as well as for the 2-pdr. The ammunition featured a tungsten penetrator and aluminum carrier, must like  the German squeeze-bore ammo. The US also experimented with Littlejohn type adapters and projectiles, and discovered that firing the Littlejohn projectile from the gun without the adapter produced equivalent velocity improvements without compromising the ability to fire  ordinary projectiles. (In effect, this was using the lightweight projectile as in Illustration A at the top of this post, rather than a squeeze-bore as in Illustration D).

langsford_extruder_bulletsFor a while, there was a squeeze bore gun that anyone could buy. Australian gunsmith Arthur Langsford, an expert in rimfire rifles, used an extended leade or forcing cone to make rimfire guns that fired an ordinary .22 LR round and produced a high-velocity .20 or .17 elongated slug. The rifling didn’t begin until after the forcing cone. They seemed to work well, but didn’t catch on, and pressure and velocity deltas between various brands and kinds of rimfire ammunition were probably larger than anything SAAMI would ever tolerate. The Myra “Extruders” Langsford made are curiosities today.

In the end, squeeze bores were a possible tank solution at one moment in time, but their performance has been overshadowed by accurate fin-stabilized discarding sabot heavy penetrators, fired (usually) from smooth-bore guns.

Next, Gain Twist, an old idea that’s making a comeback.

Sources

Department of the Navy. Naval Ordnance and Gunnery, Volume 1: Naval Ordnance .NavPers 10797-A.  Retrieved from: http://www.eugeneleeslover.com/US-NAVY-BOOKS/1-NO-10797-A-NAVAL-ORDNANCE-AND-GUNNERY.html

Langsford’s Squeeze-Bore Rimfires.Is this Near-Forgotten Idea Too Good to Die? Guns Magazine, January 2011. pp. 18-19. Retrievable from: http://fmgpublications.ipaperus.com/FMGPublications/GUNS/GUNS0111/?page=18

(Others as linked. List not completed due to time limits).

15 thoughts on “Exotic Barrels Part 1: Squeeze Bores

  1. Arturo

    Speaking of rifles, another benefit of squeeze bores that the projectile was always fully in contact with the bore. The issues of bore dimples and mistakes being reduced.

    1. Hognose Post author

      The Pak 40 in your link is a 75mm straight-bore gun. (It was one of the best AT guns of WWII and used in several German tanks, including later Pz. IVs and all Panthers. The French continued its production postwar). The squeeze bore gun was the Pak 41 which squeezed a projectile’s 75mm skirts down to 55mm (not to be confused with the 28->20mm S.PzB.41).

      (Ooops, disregard, I see you fixed the link).

      1. Y.

        later Pz. IVs

        I don’t believe so. All late-war Pz-IV’s used the 48 caliber gun. There were tank destroyers on the IV chassis that used it though.

  2. Daniel E. Watters

    Don’t forget Russell Robinson’s variant: the Salvo Squeeze Bore (SSB). This combined the squeeze-bore concept with a stack of a multiple projectiles. Colt and the US military played with the concept extensively during the 1960s and early 1970s, including combat testing in Vietnam.

  3. OBob

    For a while in the late 1800s slightly choked rifle barrels were the rage amongst the “brench rest” shooters of the day. I’ve got an 1870s 10-bore double hunting rifle with Metford rifling that has .003″ of choke in the last 3 inches of the tubes. If you tap a soft lead slug through from the breach you have to drive it all of the way; from the muzzle it will fall free after 3″.

  4. Sam

    AR15 parts manufacturer, Centurion Arms, claims that their FN made cold hammer forged barrels have a tapered bore. I have one of their upper receivers, and although I’ve never chronographed it to see if there was any added velocity benefit as a result, i found the barrel to be very accurate.

    The owner of said company was apparently a former test director for USSOCOM.

  5. S

    It’s a good thing the Germans were short on tungsten. Between the squeezies and the Pfeilgeschoss things might have turned out different…or maybe not, but still it was logistics that saved a lot of trouble, and lives, in this case especially those of our combat-oven driving brethren.

  6. Volker

    Hi
    The German name of the sub-calibre ammunition was “Unterkaliber-Munition”. As far as i know it was planned to equip the Panther Tank with a conical barrel. But they didn’t it because the abrasion of the barrels was to high for use in war times (especially to get replacements in the field).

    The “Little John” AT Rifle was an Invention of the czech manufactory Jawa. Frantisek Janecek was not an emigrant. He stayed in czechoslovakia and he died unfortunately already in 1941 by cancer.

    http://en.wikipedia.org/wiki/Franti%C5%A1ek_Jane%C4%8Dek

    Janecek restored all Schwarzlose machine guns of the czech army in the 20’s and produced fuses for hand grenades. But the big military business was made by the manufactory CZ and not by Jawa. So he decided to make motorcycles. One of his last military Inventions was the AT rifle for the czech army.

    For the development of his motorcycles he hired an english gentleman with name George William Patchetts. Patchetts turned the business into a profitable thing. When the Germans occupy Czechoslovakia in 1938 Patchett decided to go back to UK. In his luggage were the design documentations of the Janecek AT rifle. Later this rifle was developed into the “Little John” device by Mr. Patchett. Later he invented the Sterling sub machine gun introduced in the british army in the fifties under the name L2A1.

    By the way. Also the BREN rifle and all british tank machine guns were developements by czech constructors. In this case by the CZ factory.

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