For sustained fire at the rate of a million rounds /minute it would take a pretty substantial aircraft. In addition to the weight of the ammo some of Newton's laws would have a major effect on the flight characteristics during the firing.
The units are computer controlled, so firing rates are determined by need. You need to go and review the workings of the system before you write about what you don't understand.
The firing units are in big boxes and the boxes are configurations of lots of loaded barrels, you just change boxes.
A box 6' X 6' by 8 foot long could hold hundreds of barrels of 40 mm granades, with each barrle loaded with maybe 50 granade rounds. There is no shell, just the round in the barrel. Each round is fired by computer, like an ink jet printer.