I remember an article about a WWII project that modified a 1911 to fire a steel dart more-or-less silently. IIRC, the project was codenamed Bigot. The file had been declassified shortly before the article was written. Existing modified pistols and dart ammo were dumped in the ocean at the end of the war. The darts were about 6" long and were slid down the barrel of a modified 1911. A small pistol blank(.25 ACP, maybe) launched the dart, a gas check sealed the remaining propellant gas in the dart upon leaving the muzzle to silence it. A sliding collar with fins slid down to the base for stablilization upon firing. Improvements in suppressors made it obsolete; the fact that the firing pistol would have to be un-modified before it could fire regular ammo was also a problem with it. I read it years ago, many of the details are fuzzy. I couldn't find anything about it on Google.
