In this application, a revolver would be quieter than an autoloader because the autoloader’s reciprocating action adds to the total noise of the weapon (but you’d never hear it over the muzzle blast). There are commercially-available slide locking devices made for some of the more popular rimfire suppressor hosts (like the Ruger 10/22) for those occasions when you’d rather have an ultra-quiet single shot than a “sort of” quiet autoloader.
The Weapons Man (blogger) has a more extensive article on the “Quiet Special Purpose Revolver:”
http://weaponsman.com/?p=25023
The S&W forum has a ton of pics:
http://smith-wessonforum.com/s-w-revolvers-1961-1980/196939-1969-tunnel-gun-new-pic.html
Small Arms Review has a piece on it:
http://www.smallarmsreview.com/display.article.cfm?idarticles=2423
The S-A-R article claims the (tunnel-clearing) shotgun cartridge fired 15 tungsten-steel alloy pellets of 7.5-grains each (112.5-grains total weight) at 750 fps (141 fpe, if you were wondering). Noise was 120db @ 1 meter.
A cross-sectional illustration of the ‘shotgun’ version of the cartridge showing how the propellant gasses remain captured within the cartridge casing:
https://i.imgur.com/zf1ssVn.jpg
If you didn’t follow the blogger Weapons Man, you really missed something. He was an exceptional writer and a snake-eater who knew where all the bodies were buried. Regrettably, he died just a few weeks ago (at age 59) from a massive heart attack. I don’t think you could be a “gun guy” and not find his stuff entertaining, so if you’re so inclined, with his untimely death I don’t think it’s yet certain how long his blog will remain online.
Weapons man was great. I highly valued his sparse complements.
He was a wonderful writer.
