[SO April 2014 article] British Air Ministry engineer William Godfray De Lisle came up with the concept of an integrally suppressed rifle while working at home in 1942. He created a prototype chambered in .22 caliber and based on a Browning semi-automatic rifle to hunt small animals for food. After much success, he realized the extreme quiet of the rifle might fulfill his intended role for it, after all... Killing men.In early 1943, he informed Sir Malcolm Campbell, head of the secretive Combined Operations, of his invention, and Campbell suggested an interesting way to demonstrate it ‐ in the open with unsuspecting people around.
From the roof of a building overlooking the river Thames in London, De Lisle fired several shots into the water as people strode by on sidewalks. Campbell watched with interest to see if anyone heard the shots. None did. The weapon found a buyer right then. First, though, there needed to be improvements.
Combined Operations wanted a more potent caliber. Shunning rifle cartridges, a 9mm, the standard British and German pistol round, sounded right. De Lisle disagreed and suggested something larger: the U.S. .45 caliber pistol round. Denied, he created the 9mm version based on the standard Lee-Enfield bolt action. Subsequent testing showed reliability problems with the 9mm, and De Lisle, still insisting the .45 caliber the optimum choice, at last won over the officials and built the next version. This time, the tests went much better, and established the De Lisle carbine as one of the quietest firearms in history.