Mike Dillon is the only civilian I know of who is generally authorized to shoot live machinguns/cannons from an airplane. He builds miniguns for the armed forces, etc. I believe he has an L-29 that's loaded with the real stuff, and he's on film performing live fire strafing runs in the Arizona desert with it.
That said, there's usually a helicopter or two at the Knob Creek civilian machinegun shoot twice a year that hovers over the range while gunners unload their M-60's on various objects downrange (cars, washing machines, etc). Always sounded fun, but I've yet to make it.

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Rob Mears
'Surviving Corsairs' Historian
robcmears@yahoo.comhttp://www.robmears.com