The AO-63 is a Soviet two-barrel assault rifle, capable of fully automatic fire, chambered for the 5.45 x 39 mm round. It was designed by S. G. Simonov and P. A. Tkachev, and manufactured by TsNIITochMash. It can reach a maximum rate of fire of 6000 rounds/min, when fired in two-round burst mode, making it the fastest firing assault rifle known. According to some sources the weapon uses an over/under barrel configuration with a top mounted magazine insert fed by a drum magazine with a capacity of 50 or 100 rounds.[citation needed]