Developed for use as an SOE assassination weapon during WW2, the Welrod is still in use in the British Army; there are no prizes for guessing who uses it.
The Welrod was purpose built as a silenced weapon, rather than adapted, and does the job extremely well. In essence it consists of a long thick tube with a knurled twist-bolt at one end, which mounts on a pistol grip and trigger assembly, which also doubles as a five shot magazine. The pistol is loaded by twisting and pulling back the bolt, then pushing it forwards and locking it into position. When fired, the only moving parts are the trigger action and firing pin, which create a small 'ping' noise. Provided subsonic ammunition is used, there is virtually no sound at all from bullet firing. Phwooarr! Sexy huh?
The other important silenced weapon of the SOE was the De Lisle Silent Carbine.