In most pictures of an M61 Vulcan, you can see a flash suppressor (clearly not a muzzle brake, which could've made some sense to me).
Knowing that these were mostly used on US military fixed-wing aircraft, why was the flash suppressors added? On rifles, the goal is usually to prevent the operator from being blinded in low-light environments, which I doubt would be a problem for an aircraft pilot, unless I'm mistaken?
An alternate explanation would be that it's meant to make the aircraft harder to spot when firing, but I doubt that the flash suppressor would work well enough, if visual spotting during firing even is that big of a factor to take into account.