The wikipedia entry on the Hispano autocanon touches on the problem. The American showed interest in the British Hispano early on, but it appears the US manufacturers wanted to redesign the chamber somewhat. The result was that the weapon became prone to misfire. The USAF and particularly the USN had planned to phase out the .50 by mid war, but the American Hispano was delayed. Not until introduction of electrical firing post-war, did the US version become reliable enough for use in planes.
If the Americans had solved the design problems (or not redesigned the Hispano in the first place), Mustangs and Thunderbolts would have flown with 20mm Hispano canons rather than MGs. Luckily, the Americans could fall back on the .50, which gave adequate, but not great, firepower.
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