Why mount multiple autocannons on a fighter or CAS aircraft as opposed to a revolving barrel auto-cannon with a higher rate of fire?
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Same difference as between a shotgun and a rifle: if you need to hit a moving target, like a clay trap target or pigeon or aircraft, it's best to spread the projectiles out so that there is more of a chance that the target meets them. If however the target is (almost) stationary, nothing beats lining up carefully and inflicting the maximum damage with the projectile.

Heavy machine guns are mounted on swivels to create an effective area instead of a line, and only one bullet would be required to stop a soldier. A trap shooter swivels while overtaking the spinning disk, thereby spreading out the spray of shot. But an A-10 lining up for a tank kill wants to maximise impact upon this one tank only - just the one bullet landing anywhere on an armoured tank may not do the job.

When multiple cannons/machine guns are mounted across the wings, the aircraft is set up for dogfighting. Which they are not designed for anymore.