According to McDonald's MOH file, he did not participate directly in the Civil War.He was posted at Fort Union, New Mexico Territory as Acting Ordnance Sergeant.This fort served as a guardian and protector of the Santa Fe Trail and was the largest in the American Southwest. It functioned as a military garrison, territorial arsenal, and military supply depot for the southwest.
You are correct:ACS is Assistant Commissary of Subsistance (Quartermaster).
McDonald commanded Co D 5th at Tongue River Valley, Wolf Mountains in 1877.Surprised me!All his letters are signed Co E 5th Inf. There was also a line about Co K in his file, but no further information. I found the reference to Co D from the Montana Historical Society's article from The Magazine of Western History, 51 (Winter 2001), 53-67 at http://visitmt.com/history/Montana_the_Magazine_of_http://visitmt.com/history/Montana_the_Magazine_of_ Western_History/wolfmountain.htmArticle entitled "Nelson A. Miles, Crazy Horse, and the Battle of Wolf Mountains" by Jeffrey V. Pearson.
He mentioned he was in Texas during their troubles...Red River Wars?" There were fourteen battles during 1874 and 1875 with Indians. Don't know if he participated in all battles or not.