less sturdy Kjellman-style flapper locking system. These locking methods are similar in concept. By December 1943 Mauser Werke's Weapons Research Institute and Weapons Development Group had completed a roller-locked prototype rifle designated as the Gerät 03. Aside from the differing action, the Gerät 03 prototype resembled the Gewehr 43. Although the prototype rifle was machined it was designed with pressing and stamping steel components production methods in mind to simplify mass production and keep production costs low. Only a few prototypes were built and the Gerät 03 never went into production, but the Gerät 03 was put through a 5,000-round endurance trial. The Gerät 03 semi-automatic rifle used a fully locked action design with a gas system, using a gas piston to unlock. During test firing, the development group noticed an undesirable tendency in the Gerät 03 action to exhibit bolt-bounce. It was possible to unintentionally fire the Gerät 03 during the bounce phase, at which the action was not fully locked. This opened the Gerät 03 action much faster and under much higher pressure than the gas, the system was supposed to allow. This observation of a harmonics problem in the roller/wedge system led to the idea and development of the intentionally never fully locked roller-delayed blowback action design, which does not require a gas system. Shooters kits for historic rifles The original gas system of the Gewehr 43 is copiously gassed and was designed for using World War II German service ammunition in the presence of heavy fouling or icing. Being ammunition specific, the gas system was not adjustable to various propellant and projectile-specific pressure behavior and so modern (higher pressure C.I.P. conform 7.92×57mm Mauser) ammunition can move the gas piston overly fast and hard against the bolt carrier which in turn hits the receiver harder. Especially on late war Gewehr 43's the steel quality of the sheet metal rear of the receiver could be poor and the heat treatment of the bolt carrier could be suboptimal. This can lead to increased wear and damage to guns and in worse scenarios possibly endanger shooters. For shooting historic Gewehr 43's commercial "shooters kits" have been developed that basically replace the gas cup and the gas plug with new ones, and the gas plug is threaded to accept different sized gas orifices so the gas system can be adjusted to cycle reliably with the specific pressure behavior of the ammunition