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2 free books about machine guns, free to read online. Chapter 1 of any book is free, no account needed.
Machine-Gun Tactics
R. V. K. Applin
Machine gun manual: a complete manual to machine gunnery, containing details of Maxim, Vickers, Lewis, Colt, Hotchkiss, together with drill (elementary and advanced) fire orders, notes from the front and a mass of other useful information
H. Douglas