In 1875, after a brief stint as a lawman, he joined the Special Force under Captain
Leander H. McNelly, a newly created quasi-military branch of the
Texas Rangers that was to operate in southern
Texas. His role as McNelly's second-in-command and right-hand man earned him the promotion to sergeant and the nickname "McNelly's Bulldog" after the battle of Plano Alto for his marksmanship With the death of McNelly and the absorption of the Special Force by the Texas Rangers' Frontier Battalion in 1876, Armstrong was promoted to Lieutenant. He was involved in several notable cases, On August 24, 1877, Hardin claimed that he was captured while smoking his pipe and that Duncan found Hardin's pistol under his shirt only after his arrest. including the capture of Hardin and the pursuit and killing of noted bank robber
Sam Bass. Armstrong resigned from his position at the Rangers in 1881, and was appointed as a
U.S. Marshal shortly afterwards. He later established himself in
Willacy County,
Texas, where he founded a very large ranch. ==Death==