Note 18 — VII Differences in Spelling
Reading edition
Caliber is now the official spelling of the United States Army. Cf. Description and Rules for the Management of the U. S. Rifle, Caliber .30 Model of 1903; Washington, 1915. But calibre is still official in England as appears by the Field Service Pocket-Book used in the European war (London, 1914, p. viii.)