At the end of the 1930s, Krupp developed a new type of gun that utilised differential materials in its rounds to achieve considerably higher muzzle velocities (4,500ft/min compared to 2,500-3,000 ft/min for a normal gun) than could be achieved by a conventional design, which led to very high range and penetrative power of the projectile, given their relatively small size. As the gun calibre was relatively small, it was not classed as a gun, but rather a heavy anti-tank rifle, or schwere Panzerbüchse.
The weapon entered service in 1940, but despite its advantage in terms of weight and size, compared to the 5cm PAK38 gun, the appearance of the heavy Russian tank types, as well as the British Matilda II infantry tank (although there are indications that it could still threaten this tank at 600m, and it certainly could at 400m, see these trial results), meant that it was obsolete by the middle of 1941. Production was ended in 1943, with about 2,700 of the guns being produced.
Use in the Desert
Nevertheless, 1941 in the Western Desert this weapon had many things going for it. The ability to smash through 52mm of armour at 30 degrees off the vertical at a distance of 500m made it a dangerous adversary for British cruiser tanks in the Western Desert, given its small size, as demonstrated by the picture below.
The weapon generated interest with the enemy due to its unusual design, and a contemporary Popular Science Article can be found here.