According to several rumors, the AR-15 is known to be the 15th design by Eugene Stoner. Reportedly, the 15 in the name stood for the caliber size. But no! It was not. The AR-9 uses the 9 mm caliber. The 15 in the AR-15 denotes the caliber size. Jim Sullivan and Bob Fremont were two people who made the AR-10 rifle. Wanna know how to build AR-15 gun? Ping to us soon! Got any query related to the AR-15, you can contact AR-15 Gun Builder team behind them. The AR-15 is indeed an exciting weapon. It arrives in both the rifle and handgun versions.