The Mig-21F-13 is a Short-range day fighter; the MiG-21F-13 was the first MiG-21 model to be produced in large numbers. Unlike the MiG-21F, the MiG-21F-13 had only one NR-30 cannon on the starboard side, with only 30 rounds; however, it added the capability to use the K-13 missile system, of which two could be carried on underwing hardpoints. On early-production MiG-21F-13s the launch rails were of the APU-28 type; later models had these replaced by APU-13 rails. The launch rails were removable, allowing the MiG-21F-13 to carry two UB-16-57 unguided rocket launchers, two S-24 rockets on PU-12-40 launch rails or two FAB-100/250/500 bombs or ZB-360 napalm tanks. The F-13 had further upgrades: an improved ASP-5ND optical gunsight and an upgraded SRD-5ND ranging radar. The MiG-21F-13 was also built under licence in China as the Chengdu J-7 or F-7 for export, as well as in Czechoslovakia as the Aero S-106, though the S-106 designation was not used for long; subsequently, the Czech-built units were referred to as "MiG-21F-13" just like the Soviet-built aircraft.