I've noticed this a bit and can constantly reproduce. I set up multiple gun runs on a platoon of infantry fully expending my 5000 rounds. I walked my rounds THROUGH the platoon who were both standing and prone. Kills = 0.
This has been the case from the first build to now.
Set up some OPFOR in the editor, give them some HOLD waypoints with "never fire" and "careless" parameters, and try killing them with the miniguns on the AH-9.
Forum thread here --> http://forums.bistudio.com/showthread.php?161837-Accuracy-of-miniguns-on-AH-9
Others have complained of this as well.