12 pound field howitzers (not guns) used from 1.05 to 1.3 pounds of powder. The 24 pounder field howitzer used from 2.34 to 2.7 pounds of powder. The 12 pounder gun used 2.06 to 2.56 pounds. The rate of fire was entirely up to the Chief of the Piece, Gunner and crew. And of course how much fire they were taking. Firing drill and firing in combat were two different animals. Drill may have included the command "fire by detail" while in combat it was usually "commence firing". At that point the gunner would determine what rounds were delivered and their fuze time and would adjust aim and elevation accordingly. The detachment manning the gun would go through the motions. You can buy "The Red Book" which is the "1864 Field Artillery Tactics" published by the War Department. This newer volumn contained info on rifled pieces where the early version didn't. This book is LOADED (pardon the pun) with all the technical info about field artillery. I keep a copy of it and The Bible in my camp equipment box. Field Artillery, The King of Battle.