This is one of two cartridges developed by Czech engineer, Frantisek Karel Janacek between 1936 and 1941, the other being the 11/7.92mm. On the 6th of September 1915, the first British prototype tank, named ‘Little Wille’ rolled of the production line in England, and although not a roaring success, it changed the future of modern warfare. Fast forward a few decades and with Europe witnessing the War clouds on the horizon once more, armoured vehicles and the means to defeat them were being developed all over the Continent. Janacek was aware of the work done by Carl Puff, the inventor of the squeeze bore concept in 1903, as well as Hermann Gerlich. The Janacek design differed from Gerlich in that it utilised a muzzle adapter to reduce bullet diameter as opposed to the Gerlich taper bore barrel. Janacek left Czechoslovakia just prior to the German occupation and demonstrated his design to the British Military. Testing was done by Birmingham Small Arms (BSA) and the 11/7.92mm was phased out in 1939 and tests were continued with the 15/11mm only. As a result of tank armor becoming impenetrable to small arms fire, all work on the 15/11mm ceased by early June 1941 and Janacek went on to work on the 2-pounder “Littlejohn” anti-tank weapon system that achieved more success against German pantzer. See also IAA Journal #499 (Sep/Oct 2014) p. 4-17. A very big thanks to Paul Smith for his research into this.