Sources

[1] Field Artillery Gunnery FM6-40, Department of the [US] Army, January 1950.
This is the primary source of this material except where otherwise marked.

[2] David Wesely's comments and handouts from a talk on artillery at Origins'96. [Highly recommended.]

[3] Handbook on German Military Forces, TM-E30-451, US War Dept, March 1945

[4] Handbook on USSR Military Forces, TM30-430, US War Dept, Nov 1945

[5] Handbook on Japanese Military Forces, TM-E30-480, US War Dept, Oct 1944