The result of these changes was the M.S.406. The two main changes were the inclusion of a new wing structure which saved weight, and a retractable radiator under the fuselage. Powered by the production 641.3 kW (860 hp) HS 12Y-31 engine, the new design was over 8 km/h (5 mph) faster than the 405, at 489 km/h (304 mph). Armament consisted of a 20 mm (0.787 in) Hispano-Suiza HS.9 or 404 cannon with 60 rounds in the V of the engine and fired through the propeller hub, and two 7.5 mm (0.295 in) MAC 1934 machine guns (one in each wing, each with 300 rounds). A weakness of the MAC 1934 was its operation at high altitudes. It was found that at altitudes over 20,000 ft, the guns had a tendency to freeze. Heaters were added to the guns for high altitude use.
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