hardly practical for anything but Air Defense of a major base/city.

an average mobile gatling-armed unit (fighter plane, mobile air-defense gun like LAV-AD, Phalanx turret on ships) can carry from 600 rounds (in case of plane) to a couple thousand (in the case of the ship) in a single magazine, then it needs to reload.

when we divide 600 rounds by 1 million rounds/minute we get

600/1000000= .0006 minutes or .036 seconds

the full burst is less than the reaction time of a normal human (around .2 seconds) and then you are left with no ammo and a sky full of angry badguys.