The AR-15 platform rifle is the most popular rifle in the United States. The meaning of “AR” (as in the AR-15 or AR-10 rifles) has been the subject of much debate for a long time. Some folks say it means “assault rifle” or “automatic rifle.” Some of this confusion is due to ignorance and some is due to a desire to spread deliberate misinformation by gun control advocates.
Instead, the AR stands for “ArmaLite Rifle.” ArmaLite is the company that developed the rifle in the 1950s, along with the brilliant firearms design Eugene Stoner.
https://www.thetruthaboutguns.com/what-does-ar-stand-for-in-ar-15/