I must say I had never thought of that one, although CM is right - the spin may look fast but the actual velocity of the barrels about the axis is not much compared to the M/V.
Fortunately, the mechanism that activates the loading/firing/clearing of each rifle barrel is very robust and foolproof (cams inside a fixed drum surrounding the rotating cluster of barrels), so that the projectile invariably exits the barrel when it is lined up with the corresponding hole in the airplane, because anything else would be messy and inconvenient.