The 35mm calibre was found to be the optimum size for the best combination of weapon weight and effectiveness.
During the 1960s, two prototypes were produced. One was fitted with a German developed twin 30mm turret developed by Rheinmetall; the other was a turret and twin 35mm mount utilizing the 35/90 gun and developed by Oerlikon. The Oerlikon design was chosen for production and the resulting vehicle, designated Flakpanzer Gepard, was essentially a standard Leopard with the ordinary furret replaced by the Oerlikon-Contraves twin 35/90mm AA turret and associated auxiliary equipment.
The changes to suit the 35/90mm gun for the anti-aircraft tank mount were quite considerable. Ballistically there is no change, but the two guns were set wide apart, one each side of the turret instead of being as close together as possible as in the field mount. A major difference is in the ammunition feed. On the field mount the guns are magazine and hopper fed, but in the Gepard, the guns are belt fed, this being done to facilitate the quick switching from high explosive ammunition to armour piercing ammunition so that the vehicle can engage either air or ground targets at will. The barrel is also strengthened to allow for longer periods of continuous fire than might be expected from the field mount. The 90 calibre gun is designated Oerlikon 353 MK and is gas operated with a horizontal sliding breech block. The weapon housing and barrel slide on the cradle during recoil while the feed mechanism does not move. The gun can fire continuously, given a constant ammunition supply, and the theoretical rate of fire of 550 rounds per minute is the highest yet for an automatic AA weapon with single barrel.
The control equipment, Super Fledermaus, designed and built by Contraves, includes an AFR-150 impulse radar which will pick up targets within 50km range and follow them automatically inside a radius of 40 km. There is a linked optical sighting system for close engagement or emergency use. Two magnetrons allow the use of alternative frequencies if jamming is encountered. All relevant data on the target and weather conditions, etc. is fed into a computer within the system to give accurate aiming guidance. For added accuracy a continuous measurement of muzzle velocity is kept - this can change during sustained firing - and fed into the control system.