Something I've been noticing while flying is that the guns fire to the low 12 o' clock position, rather than directly ahead. I can tell because, when flying straight at a stationary ground target with the pipper exactly on it, the cannon rounds all fly over the target and hit the ground well beyond it. This is due to the aircraft's forward velocity affecting the gun's downward-and-forward velocity, causing it to not hit where the pipper is. In real aircraft, the gun is aligned approximately along the aircraft's boresight; due to the AoA of level flight changing with speed, it is impossible for a fixed gun to be always exactly aligned with the aircraft's direction of travel at different speeds, but at "normal speeds," it's going to be very close. In Arma2, however, the difference in angles is extreme, which makes aiming difficult even for an experienced shooter (this is why the real aircraft have the guns aligned close to the aircraft's direction-of-travel, rather than angled downward as in Arma2).
As I said, I was able to discern this problem very quickly simply by observing the pattern of the bullets during strafing runs, but I decided to test it to be certain. Sure enough--when fired in static tests on the ground, the guns for all fixed-wing aircraft in "vanilla" Arma2 (except for the Su-25) fire down into the ground just a few feet in front of the aircraft, rather than forward toward the general direction of the horizon. Furthermore, the angle of the actual firing line doesn't remotely match the angle of the barrel, as one can see if standing behind the barrel as it fires. Only the Su-25 fires out toward the horizon as it should, and only the Su-25's tracers approximately match the visual model of the barrel.
Curious to see if this problem was fixed in A2:OA, I looked on Youtube for videos of A2:OA A-10s flying. As far as I can tell, it hasn't been amended, judging by the HUD--the gunsight reticle is in the same place relative to the aircraft boresight indicator (about five degrees down, which may sound small, but it has a catastrophic effect on gunnery). Can anyone with A2:OA perform the ground test to confirm, and record it on video? The test is simple: fire the gun while the aircraft is stationary on the ground. If the rounds impact the ground a few feet in front of the aircraft, it's wrong. If the rounds fly out toward the horizon and hit the ground in the distance, it's (at least approximately) correct.
I do hope that this error is corrected in Arma 3, but that's sort of another discussion for the other forum.