When I was working on the short bio of Armstrong for William C. Davis's "The Confederate General," I did not find any evidence that he was ever a major. He resigned from the Union army shortly after fighting at Manassas and became a volunteer aide to Ben McCulloch and later James McIntosh. He received a commission as a lieutenant colonel and adjutant general on December 26, 1861. Armstrong continued on staff duty until named colonel of the 3rd Louisiana Infantry on May 8, 1862.