Here's an interesting one - the silent shotgun shell!
I remember reading about this a looong time ago.
[FONT=Arial,Helvetica][SIZE=-1]As a result of the problems in effectively silencing a shotgun firing conventional shotgun cartridges, the US Navy decided to try a different approach. In 1967, the Navy announced a requirement for a silent shotgun cartridge that could be fired from unmodified, conventional military shotguns. Although this may have seemed like an impossible task, AAI Corporation did in fact come up with such a cartridge, based on their patented Telecartridge™. Intended for use by Navy SEAL and Marine Recon teams, the Silent Shotgun Shell has to be one of the most unusual cartridges ever developed.[/SIZE][/FONT]
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[FONT=Arial,Helvetica][SIZE=-1]The AAI Silent Shotgun Shell utilized an explosive propellant under a folded, steel Telecartridge cup. When fired, the expanding gasses extended the cup. The payload, consisting of 12 #4 buckshot was expelled by the initial impulse. The sealed Telecartridge cartridge cup contained the gasses, flash and most of the noise.[/SIZE][/FONT]
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[FONT=Arial,Helvetica][SIZE=-1]The Silent Shotgun Shell was a great idea, but it too proved impractical. To prevent the Telecartridge from rupturing, the muzzle velocity of the round had to be reduced to just 450 fps. The low velocity resulted in a short range cartridge of limited lethality. This coupled with the high cost of the rounds caused the project to be dropped after only 200 test rounds were delivered to the US Naval Ordnance Testing Laboratory[/SIZE][/FONT]