A bolt-action, magazine fed suppressed pistol, the Welrod is purported to be the quietest firearm ever produced.
Designed for the British Special Operations Executive (SOE) the pistol was used by SOE & American OSS agents operating in Nazi occupied territory, European Resistance fighters and British Special Forces. The pistol was 'sanitised', carrying no markings other than a serial number and produced in both 32 ACP and 9mm Parabellum.
The Welrod continued in British military service with certain balaclava wearing regiments until at least the Gulf War (and may possible still be in service).
If you need to shoot people very, very quietly at close range then the Welrod is your weapon of choice.