According to Google / Wikithingy, below are the following ranges for the mentioned aircraft.
AC-130 Range: 2,500MI
C-17 Range: 6,500MI
C-5 Range: 7,200MI
I was thinking...
Has there ever been a project or proof of concept drafted to examine using a C17 or C5 in the same capacity as a AC130? Not sure if the idea is dumb or not, but from my limited observations it would seem like the other two aircraft (if outfitted properly) could deliver more rounds on target, carry larger and/or more ordance and loiter on station longer to support various mission requirements.
Perhaps there is even room to create a dual use aircraft that can do ISR as well as the orbit of death.