The Armalite AR-15, the Colt AR-15, the Colt M4, Colt M4A1, FN M4A1, and the military M16, M16A1, M16A2, are all the same weapon with minor modifications. The significant differences are barrel length (interchangeable among most if not all), upper (slick side or with forward assist, not consistent among any of them), buffer weight (generally the full-auto versions have a heavier buffer), trigger model (full auto, 3rd burst, and semi-only -- many variants exist and are interchangeable among most models w/ exception that the civilian models do not have the upper hole that allows for the full-auto trigger components to be installed).