P-63A-8, SN "269261", was extensively tested at TsAGI in what was then the world's largest wind tunnel. Soviet input was significant. With the Soviet Union being the largest buyer of the aircraft, Bell was quick to implement their suggestions. The vast majority of the changes in the A sub-variants were a direct result of Soviet input, e.g. increased pilot armor and fuselage hardpoint on the A-5, underwing hardpoints and extra fuel tanks on the A-6, etc. The Soviet Union even experimented with ski landing gear for the P-63A-6, but this never reached production. Most significantly, Soviet input resulted in moving the main cannon forward, favorably changing the center of gravity, and increasing its ammo load from 30 to 58 rounds for the A-9 variant. The P-63 had an impressive roll rate, besting the Americans' P-47, P-40, and P-51—and the Japanese Navy's Kawanishi N1K2 "Shiden-Kai" fighter—with a rate of 110° per second at 275 mph (443 km/h).