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.

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.

Silent Weapon System – Alpha prototypes at the Rock Island Arsenal Museum


  1. It's just Boris says:

    Fascinating approach. If I recall the Russians also had/have something like this?

    Given the very limited distance through which the projectile can be accelerated, strength-of-materials constraints, and practical limits on round size, I can see why a captured-piston cartridge would have trouble getting serious muzzle energy. But I have to wonder if a hybrid approach could be used. Basically something more like a piston engine: a long-stroke piston extending from an otherwise sealed combustion chamber, propellant being injected into the chamber and ignited, and pressure release performed (relatively) slowly after firing.

    Idle rambling from a tired brain.


    1. D.E. Watters says:

      Yes, the Russians have fielded several captive piston cartridges, most sized like conventional cartridges. The most prolific is probably the 7.62x41mm SP4 used in the PSS Vul semi-auto pistol, OTs-38 revolver, and NRS-2 survival knife.

      Here is a link to a video showing the PSS being fired:


      1. It's just Boris says:

        Thank you!


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