Why did two Corsair pilots fire half their ammunition into the air before a 1945 dogfight?

On May 10, 1945, two Marine pilots on a combat air patrol encountered a lone Kawasaki Ki-45. First Lieutenant Robert Klingman, Captain Jim Cox, and Second Lieutenant Frank Watson followed Captain Ken Reusser 13,000 feet upward to prepare the interception of the Nick reconnaissance aircraft. The Japanese plane was flying at 35,000 feet, well above the ceiling of the F4U Corsairs the Marines were flying that day.

On Reusser's order, the planes dropped their belly tanks, which held the reserve fuel, to climb toward the Nick they had spotted. Reusser managed to reach 38,000 feet precisely that way: by dropping his tanks and firing 2,000 rounds of ammunition into the air. Klingman also fired 2,000 rounds and likewise dropped his tanks and most of his ammunition to reach that altitude. Earlier, all four planes had reached 20,000 feet and had opened fire on the Japanese plane without success.

Reusser's Corsair came into range first and fired the rest of his ammunition at the Japanese plane, damaging the right wing and setting the right engine on fire. With his ammunition now spent, Reusser gave way to Klingman to finish the job. When Klingman lined up for the shot, he discovered that his machine guns had frozen at the great altitude.