Another sight that lists the type-18 as having 2 7.62 SHKAS and 2 SHVAK 20mm cannon. Tho I think they are wrong and the cannon never went in the 18s, unless a field modification.
Specifications:
Polikarpov I-16 Type 18
Dimensions:
Wing span: 29 ft. 6.5 in (9.18 m)
Length: 6.13 m (20 ft 1.25 in)
Height: 8 ft 5 in (2.57 m)
Weights:
Empty: 3,110 lb. (1,412 kg)
Operational: 4,034 lb (1,831 kg)
Performance:
Maximum Speed: 288 mph (463 km/h)
Service Ceiling: 29,500 ft. (8,998 m)
Range: 500 miles (805 km)
Powerplant:
Shvetsov M-62R 1,000 hp 9-cylinder radial.
Armament:
Two 7.62 mm machine guns with two 20 mm cannon.
http://www.aviation-history.com/polikarpov/i16.html