This 4mm “Zimmerstutzen” conversion kit was patented by one Karl Weiss in 1921, and produced by the RWS company in Germany. Versions were made for several different types of pistol, but the Mauser 1914 was a particularly suitable base thanks to its very easy removed barrel. The kit consists of a new 132mm long 4mm (rifled) barrel, four .32 ACP chamber adapters, a supply of 4mm zimmerstutzen cartridges, and a manual ejection rod for removing the fired cases from the cartridge adapters. The cartridge itself is basically a stretched centerfire primer with a 7 grain lead ball seated in the end. While just as accurate as .22 rimfire at short ranges, it is much less powerful, and can be safely fire with a simply pellet trap indoors without the noise and expense of shooting full size ammunition.