Why did many veteran pilots prefer the Corsair's six machine guns over cannons?

The prototype F4U-1C, BuNo 50277, appeared in August 1943 and was based on an F4U-1. Intended for both ground attack and fighter missions, it was similar to the F4U-1D, but its six machine guns were replaced with four 20 millimeter cannons. 200 examples of this variant were built between July and November 1944, all based on the F4U-1D. The version entered combat in 1945, notably during the Okinawa campaign.

Aviators preferred the standard armament of six .50 in (12.7 mm) machine guns because it offered greater advantages than cannons in ammunition supply and rate of fire, although the cannons proved effective in ground attack. Only 200 rounds of 20 mm ammunition could be carried per cannon, compared to 400 rounds per machine gun. Only 200 F4U-1Cs were purchased and the F4U-4 kept the six-.50 configuration, mainly due to persistent reliability problems with the original Hispano-Suiza cannon.

Commander J. P. Monroe stated that a 20 mm cannon was equivalent to three .50 machine guns in firing effectiveness. At 500 yards, the 20 mm could penetrate 0.75 inches of armor, while the .50 only reached 0.43. By that same reasoning, the 20 mm was lethal enough to get much more out of those 200 rounds than the .50 did from its 400.