By request, today we're going to look at one of the less common locking systems used in firearms design: flapper locking. The idea was first patented by a Swede named Friberg in 1870, but a practical gun was not built on the design until the 1907 Kjellman. The most extensive use of the system was for the Degtyarev series of machine guns (DP, RPD, DShK), although the Germans also used it in the G41 and G43 rifles. Finally, the Mauser company made a small number of early guns using the system - the 1905/06 pistol and 1916 rifle. The later roller-locked system is very similar to flapper locking, but we won't delve into that today.