There are a few of the Nagants floating around again now.
There was also a Smith and Wesson revolver developed during Vietnam for use in tunnels; it used a cartridge that looked a lot like a .38 caliber shotgun shell. The slug was set inside a long, long cartridge which extended to the front of the cylinder. When it was fired, the cartridge expanded and the front edge pushed itself forward to seal the gap. It was supposedly very quiet.