More than likely all helicopter guns suffer from this issue.
EDIT: Testing methodology -
Game version: Beta 0.71.106777 - development branch.
TEST 1: High framerate (55-60+ FPS: object distance set to 500, helicopter at "The Spartan")
Ammo Start = 5000
Ammo End = 3850
Duration = 30 seconds
Calculated Rate of Fire = 2500 rpm
TEST 2: Low Framerate (15-20 fps: Set everything to high/ultra, max view/object distance, helicopter in the middle of Agia Marina)
Ammo Start = 5000
Ammo End = 4592
Duration = 30 seconds
Calculated Rate of Fire = 816 rpm
REPEATED TESTS:
High Framerate: 2442 rpm
Low Framerate: 756 rpm
Please note: the time is measured using the IN-GAME watch alongside an IRL stopwatch. The in-game watch kept accurate time despite the differences in framerate.