Here’s a data dump on the Silent Weapon System – Alpha and related captive piston cartridges. Sadly there is more known about the ammunition than the actual firearms designed to use them.

First, we have the patent for the Alpha cartridge’s predecessor, the Cal. .30 XM76. The XM76 was intended for legacy .30-06 firearms. However, the lackluster ballistics (~.32 ACP / .32 S&W Long level) combined with the noise of manually operating the host firearm left potential users underwhelmed. The proposed equivalent in 7.62mm NATO, the XM115, was dropped quickly.

http://www.google.com/patents/US4173186

Next we have a couple of publicly available reports. Click on the titles to link to the PDF.

“Silent Weapon System Cartridge Design and Development. Phase I: Design”

“A Source of Small Arm Muzzle Noise”

“Development of a Silent Weapon System”

I think the following patent may illustrate a magazine intended for the Silent Weapon System – Alpha prototypes on display at the Rock Island Arsenal Museum. These are Items 5810-5813 in the photo below. Note that there is no chamber. They really didn’t need one, given the thick steel cases of the Alpha-series cartridges. After the Alpha1 series, they didn’t even bother adding an extraction groove to the later variants of the Alpha cartridges.

https://www.google.com/patents/US3747249