The Lee-Enfield Rifle by Major E.G.B. Reynolds, 1960. Book was written with full access to original paper before it was shredded.
Shoot to Live! - The Johnson Method of Musketry Coaching, 1944. Remains today the finest single book on accurate shooting with the Number 4 Rifle; most is applicable to many other rifles.
You can download both of those, and a library of others, from the Military Knowledge Library over on milsurps dot com. There is a Sticky at the top of this Forum to it. Mre information than you can ever assimilate..... and it's all FREE.
Text Book of Rifles -1909, His Majesty's Stationery Office, London. Rare and very expensive but utterly authoritative. Diagrams are irreplaceable, chapters on external and internal ballistics are wonderful. I have an original copy and am scanning it preparatory to having it posted for free download at milsurps dot com. This book gives you the exact state of the world's military firearms just prior to the outbreak of the Great War.
For ammunition, any of the British military TEXT BOOKS or POCKET BOOKS are Gospel as far as they go.
For Ammunition Development, the Absolute Authority was the late Major Peter Labbett. ANYTHING he wrote is utterly authoritative, partly because he was personally involved with much of that development, although he did not mention this in his books (he did in private letters). His writing style is almost incredibly condensed but the information is pure gold.
And, of course, Hatcher's Notebook by Maj/Gen Julian S. Hatcher. Available in 3 editions since 1948, the 3rd recently reprinted, it is absolutely authoritative on a myriad of difficult questions. Hatcher numbered among his personal friends such luminaries as John Garand and Melvin Johnson; he was the man who identified the problem with the 1903 Springfield and later condemned a million rifles.
And, of course, THE GUN AND ITS DEVELOPMENT by W. W. Greener, 1910. Utterly essential.
Hope this helps.
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