It is generally known that John M. Browning never retired, instead working on firearms design right up until the day of his death in 1926. His final gun would eventually be completed by Dieudonné Saive and become known as the FN High Power. Well, Browning wasn’t a single-task sort of guy. While he was working on the High Power pistol, he was also waiting for a couple patents to be granted for new feeding devices for his M1919 machine gun.
By the mid 1920s, Browning saw ahead to the coming use of his M1919 machine guns in aircraft, particularly bombers and transports. Mounting a machine gun in a fixed position on a fighter is a relatively simple procedure, but a flexible mount in a bomber presents some challenges. For a belt-fed gun, it is necessary to either have the whole belt move with the gun, or provide a flexible guide to feed the belt when the gun is in various positions. Browning had a couple ideas to provide large-capacity magazines mounted on the 1919 to avoid this problem. In July of 1925 he applied for a patent for a top-mounted drum-type magazine (like the Lewis or DP), and a few months later in January of 1926 he filed an application for a patent on a saddle-type magazine (like the MG15 and MG34).
Both patents were granted to Browning, but not until after his death (the drum in 1927 and the saddle in 1931). Neither device was ever manufactured, to the best of our knowledge, probably because Browning could not personally push the concept.
Would these magazines have made a significant difference in machine gun use in World War II? Probably not a huge one, but we can see the concept being useful in aircraft and perhaps on the 1919A6 light machine gun. They would have been neat, that’s for sure. You can download both of the complete patents here:
Drum magazine: US Patent 1,629,652 (J.M. Browning, “Cartridge Feeding Mechanism for Automatic Firearms”, May 24, 1927)
Saddle magazine: US Patent 1,800,595 (J. M. Browning, “Magazine Feed Mechanism for Machine Guns”, April 14, 1931)