In these videos the spread of the guns can be seen moderately clearly.
http://youtu.be/liAnvVyf4e8?t=11s
http://youtu.be/c0GyZkawz3s?t=12s
while it is a different caliber gun (most likely .50cal) and also most likely not a rotary cannon, the idea is the same. No cannon fires with laser-accuracy.
In this video, we can see footage from DCS: UH-1H Huey. The Huey uses the same type of guns, the M134 7.62 gatling guns, so the spread pattern in Arma should look more like this:
http://youtu.be/LUTzO4WQgSo?t=1m53s
(Note that the engagement ranges seen in this video are much larger than the typical Arma engagements, so spread at most ranges would not be as exaggerated as this video shows).