This was an unsuccessful high velocity experimental cartridge by Rheinmetall-Borsig designed for aircraft machine guns. It appears to have been a parallel development with the 13mm machine gun. At that time during WW2, the Germans were on the defensive and were taking serious damage by the constant bombing raid from the British by night and the Americans in their B-17’s. the biggest problem was that not enough explosive/incendiary compound could be fitted into the small 7.92mm bullets. The American B-17 was fast and heavily armed and it took significant firepower to bring them down, something the smaller bullets were incapable of doing.