The prototype F4U-1C, appeared in August 1943 and was based on an F4U-1. A total of 200 of this variant were built from July to November 1944; all were based on the F4U-1D and were built in parallel with that variant. Intended for ground-attack as well as fighter missions, the F4U-1C was similar to the F4U-1D but its six machine guns were replaced by four AN/M2 cannons with 231 rounds of ammunition per gun. The F4U-1C was introduced to combat during 1945, most notably in the Okinawa campaign. The firepower of 20 mm was highly appreciated. It was believed that the 20 mm cannon was more effective for all types of combat work than the .50 caliber machine gun. However, despite the superior firepower, many navy pilots preferred .50 caliber machine guns in air combat due to jam and freezing problems of the 20mm cannons. These problems were reduced as the ordnance crews gained experience until the performance of the guns compared favorably with the .50 caliber, but freezing problems remained at until gun heaters were installed.