After Fairchild sold the AR-10 and AR-15 designs, Armalite could no longer use direct-injection gas systems on the rifles. So Eugene Stoner began designing a rifle using a short-stroke piston transmission. The structural change from forged aluminum to stamped steel in the AR-10 and AR-15 allowed the new design to be easier to produce. Since research into small-caliber high-velocity rifles has not yet been approved, Stoner's AR-16 still uses 7.62 × 51 mm ammunition. When Armalite was reorganized, Stoner left the company. The U.S. military later decided to use the AR-15, which fires a 5.56mm caliber, and Armalite began looking for a design that could rival the AR-15. Due to Stoner's departure, the task of reducing the caliber of the AR-16 was handed over to Armalite's new chief designer: Arthur Miller. The resulting design was designated the AR-18. Arthur Miller patented this rifle in 1969.