The De Lisle carbine was a British firearm used during World War II that was designed with an integrated suppressor. That combined with its use of subsonic ammunition, made it extremely quiet in action, possibly one of the quietest firearms ever made. Few were manufactured as their use was limited to specialist military units.
Designed by an Air Ministry engineer, William Godfray de Lisle, the primary feature was its extremely effective suppressor, which made it very quiet in action. So quiet that working the bolt, makes a louder noise than firing the cartridge. The suppressor, taking up half of the overall length, provided a large volume to contain the gasses produced by firing, which was key to its effectiveness. Howver, while the carbine was silent, it was not very accurate. Chambered in .45 ACP, the cartridge was selected due to its muzzle velocity being subsonic for typical barrel lengths.