GAU-8 Avenger
Effective firing range 4,000 feet (1,220 m)
Maximum firing range Over 12,000 feet (3,660 m)
Of course air resistance is a factor, but the depleted uranium rounds that thing fires are mighty dense compared to their cross-sectional area, and the round would spend quite a while at high altitudes where the air is pretty thin (apogee with no air resistance would be a bit more than 107,000 feet). So it might fly an appreciable fraction of that 78 miles.
I think that 12000 foot "maximum firing range" figure is probably more like "maximum range for any practical shot at an actual target."