Charles Talleyrand wrote:

Has anyone used a helicopter cannon at LONG standoff range. By long range I mean
a range where the gun must shoot significantly above the straight line to the target
rather like a howitzer.

My vision is a helicopter standing off for safety and firing at an area. I'm assuming
if an AH-64 unloaded it's magazine at me from 10 miles away I would experience
a hail of shells all around me that would chew up every soft target including
any anti-air batteries.

It would take some sensor to measure range accurately (laser rangefinder) and some
software to compute tragetories, but these things need not be heavy or very expensive.

Has anyone ever even experimented or studied such an idea?

First the gun can't shoot 10 miles. Second, You can't tell friend of foe from that far
out. If you want to reach out an touch some one, you call on a fire mission. Not a
Apache, wasting ammo.