data into the sight. The loaders feed clips into the twin autocannon manually as needed, similar to the Bofors 40mm. Armament The twin S-68s are recoil-operated and weigh 4,500 kg. Their construction was based on two 57 mm S-60 AA autocannons. The guns have a recoil of between 325 and 370 mm. The individual weapons cannot be swapped from one side to the other as they are mirror images. Each air-cooled gun barrel is 4365 mm long (76.6 calibers) and is fitted with a muzzle brake. They can be elevated or depressed between −5° and +85° at a speed of between 0.3° and 20° per second, the turret can traverse 360° at a speed of between 0.2° and 36° per second. Drive is from a direct current electric motor and universal hydraulic speed gears (a manual mechanical drive is also provided in case of electrohydraulic failure; with the use of mechanical drive, elevation speed is 4.5° per second and the turret traverse speed is 4° per second). The guns firing together are capable of firing up to 210–240 fragmentation and armour-piercing tracer (AP-T) shells per minute, with a practical rate of fire of between 100 and 140 rounds per minute, which gives a fire time of 2-3 minutes before running out of ammo. Muzzle velocity is 1,000 m/s. Each clip has 4 rounds, each of which weighs 6.6 kg; the charge in each round consists of 1.2 kg of 11/7 nitrocellulose powder, a projectile weighs 2.8 kg. Maximum horizontal range is 12 km (with an effective range against ground targets of up to 4 km / 2.5 miles. Maximum vertical range is 8.8 km with a maximum effective vertical range of 4.5 km / 14,750 ft). Fragmentation rounds have a safety-destructor which activates between 12 and 16 seconds after being fired to ensure the shells won't fall back to ground, so the maximum slant range of anti-aircraft fire is 6.5–7 km. BR-281 armour-piercing rounds are able to penetrate 110 mm armour at 500 m or 70 mm armour at 2,000 m (at 90° impact angle). The S-68 autocannon was the most powerful AA gun installed on SPAAGs at that time. According to the statistical data of the Air Defence Research Institute No. 2, a direct hit of a single 57 mm shell could destroy a contemporary jet aircraft. In order to shoot down a jet bomber of the Canberra type,