The AO-63 is a Soviet two-barrel assault rifle 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 uses an over/under barrel configuration, and 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 a top mounted drum magazine with a capacity of 50 or 100 rounds.
The AO-63 assault rifle was used by the Spetsnaz during the "Abakan" trials in the mid-1980s, in search of a more accurate alternative to take place over the standard issue AK-74, with AN-94 being the winner among numerous other weapons.