First some notes on the two legendary Aircraft.
Japanese Zero wrote:When it was introduced early in World War II, the Zero was the best carrier-based fighter in the world, combining excellent maneuverability and very long range. In early combat operations, the Zero gained a legendary reputation as a "dogfighter", achieving the outstanding kill ratio of 12 to 1.
P51-Mustang wrote:The North American Aviation P-51 Mustang was an American long-range single-seat World War II fighter aircraft. Designed and built in just 117 days to a specification issued to NAA by the British Purchasing Commission, the Mustang first flew in Royal Air Force (RAF) service as a fighter-bomber and reconnaissance aircraft before conversion to a bomber escort, employed in raids over Germany, helping ensure Allied air superiority from early 1944.
The scenario, a Summer morning in the Pacific Ocean, 1944, both aircrafts are launched from carriers and meet in the skies above. Who will win?
Honestly, I can see how either aircraft would win. The Japanese Zero had a larger barrel size on it's gun (20mm as opposed to the Mustang's 12.7 mm M2 Browning machine guns, and while the Mustangs were excellent fighter aircraft (found great use as an escort), the Japanese Zero were designed to BE dog fighters. However, the Mustang did have a better engine and better aerodynamics, which would give it an advantage.
I'd have to say, in a knock out drag out dogfight, the Mustang would win, mainly because it did have a better engine and fuselage design. The 20mm cannon won't do you any good if you can't swing your aircraft around fast enough to aim.