Waisted barrel: a forgotten branch of German anti-tank artillery
7,5 cm Pak 41 (Panzerabwehrkanone 41)
In a conventional cannon, the projectile travels most of its way down the barrel under decreasing pressure. The propellant burns within the first centimeters, the gases begin to expand, and the volume behind the projectile grows faster than the projectile itself can accelerate under their influence, and by the time it reaches the muzzle, the acceleration is already due to inertia. Is it possible to make the projectile narrower along the way, so that the pressure is maintained longer? In 1932, German engineer Hermann Gerlich answered: yes. Then the fun begins. story an engineering idea that worked perfectly until it was caught up with by a shortage of raw materials and the logic of mass production.
Geometry as an accelerator: what Gerlich invented
Gerlich was not a military designer. In the 1920s, he worked on hunting rifles, searching for a way to accelerate bullets to speeds unachievable with conventional rifles. The idea he came up with, which he patented in 1932, was simple: the barrel should taper from the breech to the muzzle, and the bullet should have soft bands that would compress along the way. His hunting rifles were not commercially successful: they were too expensive and too finicky. But the patent remained.
Gerlich's bullet for conic trunks
By the late 1930s, this idea had already been revived in military logic. To understand the advantage, two things must be kept in mind. First, the pressure in the barrel after the propellant charge burns out drops as the projectile moves forward, the volume behind it increases, and the gases expand. In a conical barrel, this volume increases more slowly, the projectile narrows along its path, and the pre-projectile space expands less rapidly than in a cylindrical tube of the same diameter at the breech. The pressure remains higher for longer.
The second is the strength of the barrel. The breech walls are thick and can withstand high propellant gas pressure; the muzzle has a smaller diameter, and with the same internal pressure, the stress on the walls is lower. In a conical barrel, the pressure drops less sharply as the projectile travels than in a cylindrical barrel, but the walls are also thinner towards the muzzle, so the reduced pressure is still manageable. This allows the initial pressure at the breech to be raised higher than in a conventional gun of the same caliber and weight. Together, these two factors give the projectile a significantly longer section of intense acceleration.
For this to work, special ammunition is needed. The core is hard, dense, and relatively small in diameter, usually made of tungsten carbide. It is surrounded by a clip with soft, light-alloy bands that fill the entire caliber at the breech, but as they pass through the conical section, they are crushed, bent inward, and partially sheared off. At the muzzle, the projectile flies a narrow, heavy core with the remains of the bands: the mass is almost the same, but the velocity is significantly higher.
There are two ways to implement this design. First, make the barrel conical along its entire length—expensive and complex, but provides maximum effectiveness. Second, keep the barrel as is and screw a short conical adapter onto the muzzle. The British later followed this path with their Littlejohn adapter For a two-pounder gun: a narrowing from 40 mm to approximately 30 mm, mounted on a standard gun. The increase is more modest, but there's no need to redesign the barrel. The Germans chose a more difficult route.
2,8 cm sPzB 41 captured by the British on the African Front (6 March 1942)
2,8 cm sPzB 41: small caliber, high velocity
The first system adopted for service based on the Gerlich principle was 2,8 cm schwere Panzerbüchse 41Formally a "heavy anti-tank rifle," it's actually a fully-fledged small-caliber cannon with a carriage, shield, and crew. The name comes from an era when it wasn't clear how to classify a hybrid: too big for a rifle, too small for a cannon.
The barrel tapered from 28 mm at the breech to 20 mm at the muzzle. The gun weighed approximately 220–230 kg; a motorcycle with a sidecar was sufficient for towing, and the crew manually moved the system once in position. The ammunition was a shot with a tungsten core; the muzzle velocity, according to German data, reached 1400 m/s. For 1940, this was incredible.
The tactical niche was self-defined: paratroopers, scouts, light infantry. The gun could be concealed in any ravine, firing from ambush at the sides and rear of light and medium tanks. tanks, then changed position. At ranges of up to 300–400 meters, the sPzB 41 was confidently effective against armored targets, for which a standard 20mm automatic cannon was already too weak. According to crews, when hitting the side of a light tank, the effect was "like a good anti-tank grenade, but from five hundred meters."
2,8 cm heavy anti-tank rifle sPzB 41 (German: 2,8 cm schwere Panzerbüchse 41) from World War II
The ceiling was immediately visible. The projectile was light, and its energy dropped off more quickly with distance than that of heavier calibers. By 1942, the system had ceased to be a threat to the front of medium tanks; a high-explosive fragmentation shell of this caliber was only useful against a machine gun nest. Most importantly, each armor-piercing shot wasted grams of scarce tungsten. The gun remained in service, but quickly transformed from a mass-produced anti-tank weapon into a specialized tool for those for whom compactness was paramount.
From 4,2 to 7,5: How the principle was scaled
Logic further dictated a caliber increase. If a conical barrel increases velocity, then the effect should be maintained even with a larger core, and armor penetration at longer ranges would increase with the projectile's mass.
German 4.2 cm Pak 41 (Panzerabwehrkanone), used by airborne troops on the Eastern Front during World War II.
4,2 cm Pak 41 Externally, it was almost indistinguishable from the standard 3,7 cm Pak 36: the same carriage, the same shield, the same layout. The difference was inside the barrel: 42 mm at the breech, about 28 mm at the muzzle. The idea was to replace the Pak 36, which the Germans themselves sarcastically dubbed the "Pak 36" after their first encounters with the T-34 and KV Heeresanklopfgerät ("army door-knocking device"), for a system of the same weight but with radically different armor penetration. According to consolidated data, about 300 were produced, and by November 1943, fewer than fifty remained in service. The barrels wore out, new ones were expensive to make, and the supply of special ammunition was uneven.
7,5 cm Pak 41 It was the culmination. A 75/55 mm barrel, an armor-piercing projectile with a tungsten core weighing approximately 2,6 kg, and a muzzle velocity of approximately 1260 m/s. For comparison, a standard Pak 40 of the same caliber accelerated its armor-piercing projectile to approximately 790 m/s. A difference of almost 500 m/s with a similar system weight. Exactly the increase in velocity that made the whole conical barrel thing worthwhile. A captured Pak 41 prototype was tested in the USSR; according to these tests, the gun penetrated a 120 mm plate at an angle of 60° at a range of 500 m.
75-mm anti-tank gun Pak 40 (Panzerabwehrkanone 40
The paradox is that this same Pak 40, a conventional 75mm cannon with a cylindrical barrel, no tungsten in the ammunition, and no complex bore geometry, ultimately killed the Pak 41 program. Although the Pak 40 was significantly inferior to the Pak 41 in armor penetration, it was superior in every other respect: cheaper, simpler, without tungsten, capable of firing a full range of projectiles from armor-piercing to full-fledged high-explosive fragmentation, and its barrel lasted a long time. The Pak 40 ultimately carried the weight of the Wehrmacht's entire anti-tank defense system—more than 23 were produced. The Pak 41 produced only a few hundred, after which the program was abandoned.
Solution Cost: Tungsten, Wear, Logistics
If you look at why it all ended, the picture emerges from three components, and none of them are connected to the fact that the Gerlich principle "didn't work." It did work, and well.
The main issue came down to tungsten. Tungsten carbide was a prerequisite for the stated performance: a steel core of the same geometry at the same speeds would simply shatter against armor without penetrating it. Germany imported tungsten from Portugal and Spain through complex channels; by 1942, supplies were dwindling, and the metal was needed not only by artillery: without it, toolmaking, cutting machines, and milling cutters would grind to a halt. In June 1942, a directive was issued sharply limiting the use of tungsten in ammunition. For Herlich-based systems, this was a death sentence: no alternative material with the same properties existed.
A - A conventional gun firing light full-caliber projectiles
B - A conventional gun firing shells with a sabot
C — A long-barreled, high-strength weapon firing standard projectiles
D — A gun with a conical bore, firing projectiles with a "skirt" (belts)
E — A conventional weapon that fires rocket-propelled projectiles.
Added to this was wear. The conical section of the barrel operated in a manner uncharacteristic of a conventional cannon: under high pressure, the projectile's soft bands didn't simply slide, but rather deformed plastically, abrading the bore surface. The service life of the Pak 41's conical section was estimated at 600-1000 rounds before a noticeable drop in accuracy and armor penetration. For a field gun capable of firing hundreds of shells a day on the front lines, this was extremely short: the barrel's entire service life was exhausted within a week and a half of fighting. Some systems were designed with replaceable conical sections while industry could still afford such delicate work. By 1943, when production was transitioning to simplified technologies and expanding serial production, conical rifling was considered a luxury. The same machine tools could be used to make barrels for the Pak 40: faster, cheaper, and without tungsten in the shells.
The Gerlich principle in its purest form ended there. The design remained in the 1940s. The problem of accelerating a dense core to superspeed and delivering it to the armor hasn't gone away; it's being solved by discarding sabot subcaliber rounds, and has been for a long time. The conical barrel in this family turned out to be a side branch—a short-lived and dead-end one.
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