Reaver:
Thanks for reminding me. I'd forgotten all about that.
The U.S. Army "Tunnel Rats"-- a branch of the U.S. Army's 1st Batallion, 28th Infantry Division, which was itself a branch of the 3rd Brigade of the 1st Infantry Division ("The Big Red One"), developed a special handgun for use by their personnel in Viet Nam. It was called the "QSPR", or "Quiet Special Purpose Revolver", but was also known, in their typical backwards jargon, as the "RQSP", or "Revolver, Quiet, Special-Purpose". I first read about it in GUNS & AMMO shortly after the end of the U.S. involvement in Southeast Asia, which would've been almost thirty years ago.
It was a highly modified Smith & Wesson Model 29 .44 Magnum. The conventional, rifled barrel had been replaced with an unrifled one which was about two inches long, and there were no sights--the weapon was strictly a close-quarters, "point-and-shoot" pistol.
The ammunition resembled conventional .44 Magnum cartridges, except that they were .53-caliber, instead of .44-caliber, and were made of blued steel, instead of brass. There was a thin steel nose cap, held in place by waterproofing sealant.
Inside the cartridges were the typical primer and gunpowder arrangement. However, forward of the propellant was a brass piston, and in front of that were 15 tungsten pellets, each of which was approximately the size of a Number 9 birdshot pellet.
Upon firing, the interior piston moved forward propelled by the gunpowder explosion, shoving the pellets out at moderate velocity--about 800 to 900 feet per second. When the piston reached the end of its travel, it sealed the case mouth, trapping the remaining propellant gases (and hence, most of the noise and muzzle blast; the latter was handy since the Tunnel Rats usually worked in near-darkness) inside the cartridge case. About the only noise heard was the mechanical noise of the revolver's mechanism (the loudest part of which was probably the hammer falling) and the muted "pop" of the cartridge's primer going off. I've obviously never heard one fired, but based upon that information, I would suspect that it wasn't much louder than the report of a child's cap pistol.
There were, however, a few disadvantages to this system.
One was that, being essentially a small-bore, VERY short-barrelled shotgun, it didn't have much range. Fifteen to twenty feet (yes, I said feet, not yards) was about the best you could expect for decent accuracy.
Another was that, due to the interior piston and the external nose cap being virtually identical (except for color and construction material, that is), it was virtually impossible to tell a fired case from an unfired round of live ammunition without examining the primers. Possible in the dark, I suppose, but it would require a better-than-average sense of touch if the user didn't want to give himself away.
I've seen a photograph of the revolver in question, or at least a prototype, but I'm not certain if any were actually fielded or used in combat.
Some may be in military museums, or possibly in the hands of private collectors, although I doubt the latter very much. A private citizen would have to jump through quite a few legal hoops in order to purchase such a weapon, as it technically (because of its size and the unrifled barrel) fits the legal definition of "sawed-off shotgun" under Federal law, which is (paraphrasing) "a firearm with an unrifled barrel of fewer than sixteen and one-half inches in length and/or an overall length of fewer than twenty-six inches".
Such weapons are called "Class III Firearms or Devices" (the term "devices" applying to sound suppressors or silencers) in the U.S.A., and must be registered with the authorities (usually the Federal Bureau of Alcohol, Tobacco, Firearms, and Explosives--originally part of the Department of the Treasury (as the BATF, or "Bureau of Alcohol, Tobacco, and Firearms"--when you hear folks in the old movies, such as
THUNDER ROAD, talking about "revenooers", these are the people to whom they refer, even if they themselves don't know it), now part of the Department of Justice, under the auspices of the Department of Homeland Security) if purchased by civilians. I'm not certain whether the QSPR would fit the legal definition of a silenced handgun since it is, in actuality, a handgun firing silent ammunition. In other words, possession of either the revolver or the ammunition
separately would most likely be legal, but definitely NOT both at the same time without the approval of the appropriate Federal, State, and Local law enforcement officials.
Anyway, I see no reason why the same principle could not be used with conventional shotgun ammunition, even including flechette or slug rounds. After tooling and everything, it should be less expensive to make in the long run than the 20th Century, .44 Magnum-based cartridges because there's no miniaturization involved. Also, with the advances in weapons technology, a more powerful shell could probably be used, along the lines of the nominally 12-gauge Close-Assault Weapon System (CAWS) that our military was working on a few years ago. I say "nominally 12-gauge" because the shells were the same size as a standard 12-gauge 2.75" shell (because the CAWS could, if necessary, also use standard ammunition), but the 12-gauge CAWS shells were loaded to much higher operating pressures--high enough to cause a standard shotgun to come apart.
Hope this helps.

--Foreigner