9. Hogg, The Machine Gun, pp. 26-27; DeWar, The Great Munition Feat 1914-1918, pp. 83, 193-194; Crowell, Benedict and Wilson, Robert Forrest, The Armies of Industry, Englewood, New Jersey, Yale University Press, 1921, pp. 204, 405

Why 47 rounds? The Lewis drum loads in a spiral. The last three slots form the feed ramp into the weapon. So the last three spaces do not have a round. It has nothing (as is commonly believed) to do with jamming but simply because the magazine would not hold those three rounds. The buttet points fit into a groove in the centre of the magazine. The last three slots are open, thus allowing for feed into the weapon. The magazine could not physically have held 50 rounds or 100 rounds (in the case of the aircraft weapon). If you tried to load those three slots the rounds would fall out. When removing a magazine from a weapon that still has ammunition those three rounds fall out and have to be re loaded.

For this information, the author is indebted to Joe Sweeney (blimey1915@aol.com)