K bullet

The German Army first employed a "reversed bullet" with a heavier powder charge behind it as an early anti-tank method against the British heavy tanks of World War I.

This did prove effective against the thinner-armored Mark I tank models, mainly by causing spalling from the inside surface of the armor hit.

Reversed bullet loads were however unsafe to use in the standard issue Mauser rifle and thus were unpopular with German troops.

The K bullet was in use by the Battle of Messines Ridge in June 1917 and had a one-in-three chance to penetrate 12–13 mm thick armor at a range of up to 100 meters.

[4] With the British deployment of the Mark IV tank, which had more armor thickness, the K bullet soon proved ineffective, leading the Germans to design highly specialized anti-tank solutions, with the creation of the powerful 13.2mm TuF cartridge and the first anti-tank rifle, the Mauser 1918 T-Gewehr.