The ZK-381 was designed by Josef Koucký, his first design of 1938 (hence 381). This is one of the last of the Czech pre-war self-loading rifle projects, of which there were quite a lot. It uses a tilting bolt and a short-stroke gas piston, with ZB26 machine gun magazines and chambered for 7.92mm Mauser (although they would have been happy to offer a model in any other modern rifle cartridge). It was tested in the spring of 1938 by the Soviet Union, which liked it enough that they requested a model in 7.62x54R - and those were tested in November 1938. Ultimately domestic Russian designs were chosen instead, and tests in German, France, Spain, and Italy also led to naught.

Thanks to the Czech Military History Institute (VHU) for graciously giving me access to this one-of-a-kind prototype to film for you! If you have the opportunity, don't miss seeing their museums in Prague:
https://www.vhu.cz/en/english-summary/