When the Luftwaffe was looking for its new universal paratrooper rifle, six different German arms companies were asked to submit proposals. Only two actually did; Krieghoff and Rheinmetall. Krieghoff designed this very interesting system, clearly optimized to reduce weight and length as required by the design brief. It uses a tiny vertically traveling locking block and an unusual gas trap system combined with an under-barrel piston. The total number made is unknown, but both fixed- and folding-stock models were produced (the German museum at Koblenz has a fixed-stock example on display). This particular example appears to have been tested after the war by engineers at Springfield Armory by drilling a hole in the gas tube to measure pressure while it cycled.

Thanks to the Springfield Armory National Historic Site for giving me access to this rare prototype from their reference collection to film for you! Don't miss the chance to visit the museum there if you have a day free in Springfield, Massachusetts:

https://www.nps.gov/spar/index.htm