The Sauer Amboss (Anvil) was a OOAK German prototype pistol in 9mm but which fired special SS-designed hypercompression rounds, the mixture of which was lead-azide and another compound which were supercompressed into tiny balls coated in a red substance and dried. The bullet tip was connected to an internal ignitor that detonated the compression balls. Some accounts say that the 9mm produced 40mm strikes against concrete, light armor, and other solid objects. Others say that the test rounds were so powerful that they created visible shockwaves when detonated! Sauer's gun was one prototype meant for Waffen SS limited issue. Other specials were Flakgas guns taken from the Austrians in 1938 and further developed by the SS and rocket guns firing rocket rounds. All of these are prototype weapons reported in German and Allied Intel papers. But none have any photgraphic record except for the rocket gun rocket bullets. Hansler's 40mm electric gun has the schematic and so does the HF 15 helical cannon, but no actual photos of the prototypes as built and/or captured. If anyone ever locates a photo, let me know... same for the Colt Arbitrator .45!

A "P.O. Box" just prior to the end of WW2 was an independent Intel group assigned to specific German secret weapons considered BEYOND 1940s technology. Each group operated from P.O. Boxes only- no Intel designation given. German tech included: Flight Discs, spacecraft, EM weapons, Rail Guns, and anything to do with weapons manufactured using quantum physics, Tesla-tech, and German Metaphysical applications which were not understoood by the Allies. The compression tech here would warrant P.O. Box classification under unusual firearms tech.