Why did the US Navy use the 20 mm Oerlikon rather than the Army's Hispano-Suiza cannon?
Short answerThe Oerlikon tolerated sustained high-volume fire much better, while the more precise Hispano was optimized for shorter aircraft firing cycles.
NotesThe Hispano-Suiza is characterized as a finer and somewhat more precise weapon, well suited to short aerial engagements. Aircraft could fire a limited ammunition load and then receive maintenance after landing. A shipboard anti-aircraft gun instead needed to accept thousands of rounds and keep working during repeated attacks. The Oerlikon's ruggedness and sustained volume of fire therefore mattered more at sea than the Hispano's advantages in an aircraft installation.