On July 13th, 1940, a series of trials of the 2.8 cm schwere Panzerbuchse 41 anti-tank gun began at the Kummersdorf proving grounds in Germany. This was the first ever application of a conical barrel in artillery.
The principle of a conical barrel and a bullet for it were developed by a German gunsmith Karl Puff between 1903 and 1907. In 1920, another German engineer, Hermann Gerlich, refined the design.
The conical barrel's diameter next to the breech is larger than the diameter at the muzzle. A shell used in a conical barrel has special guiding bands that compress as the barrel width decreases. Because of this, the expanding gases act on the shell optimally throughout its presence in the barrel, and their energy is most efficiently used. The muzzle velocity of a shell of bullet released from a conical barrel is much greater than one released from a regular barrel. This increases penetration, and allows the use of a lighter shell.
Initially, conical barrels were only used in small arms. In 1930, Gerlich was testing a rifle he designed with a muzzle velocity of 1700 m/s. From 50 meters, it effortlessly penetrated a 12 mm armoured plate. In 1932, Gerlich, having emigrated to the United States, proposed his new rifle to the US Army. Its bullets were fired at a speed of 1800 m/s, a record that stands to this day. The rifle did not make it into production due to high cost and complexity.
The use of conical barrels in artillery became possible when it could be established that a shell retains sufficient stability in the cylindrical rifled section of the barrel, after which it can be compressed in a conical section. Due to this discovery, the conical section of the barrel could become smoothbore, which made the gun simpler to produce. Guns with conical barrels were used in the German army and had very high technical qualities.
In the USSR, work on a conical gun began before the Great Patriotic War, but Soviet engineers were unsuccessful. After studying captured German weapons, Soviet engineer Grabin created a light anti-tank S-40 gun, using the German 7.5 cm PaK 41 as a prototype. Grabin's gun had superior ballistics and penetration compared to the German gun, and was capable of penetrating any enemy tank at any combat distance at the time.
Original article available here.