I gave it some thought.. what the 'target drift' mentioned before, might be.. There were some explanations from Hohun during the thread, but I failed to get an answer from Mr.Williams who first brought it up - wish he'd be back to clarify this.
Okay, so basically, this is what I imagine.
I'm thinking of something like what happens to a person holding up a bow or a gun in archery or shooting contests.
Basically, like mentioned by many other pilot accounts, I don't think the 'nose bounce' messing up aim is realistic. Or rather, upto a certain point.
Unless a plane is badly out of trim, or a sim depicting that aspect is wildly exaggerated, I wouldn't think moving the nose of one's plane to a certain general direction would cause the bobbing around - like, when you hold up your bow or a gun, your arm stays rock solid, you hold it firm. Your arms don't shake up and down like a scarecrow - which, in a simulated plane, would be equivalent of the nose bobbing up and down with each stick input.
But, in archery(I dunno about gunnery - guns out of special purposes are illegal in our country, and we don't have shooting ranges), what differentiates the expert marksmen and a poor marksmen is how long you can remain absolutely immobile as you aim and let go of the arrow.
When you aim a point, there are minor movements, shakes, trembling of your arms caused by many things such as tension and breathing. So, while the general direction of where you hold up and point your weapon lies pretty much immobile, minor and small movements still happen continuously, influencing your point of aim.
If you don't concentrate while holding up your weapon, you'll soon realize that the point where you are aiming at right now, has actually moved from your original targetting point. Very small movement, but significant enough to effect the outcome of the aim.
So, maybe the "target drift" mentioned, or issues on "flying on rails" is something like that..??
In the original IL-2, there admittably was difficulty in aiming through what Mathman has mentioned in this thread - the super sensitive trim issues kept attempts to keep the general heading of the plane itself unstable. Also, the high loss of E and generally low speed/acceleration of the planes, left fighters in IL-2 flying at pathetically low speeds - 1944 fighters, in general combat, were flying around at something like 300~400km/h.. slight contest of maneuvering often resulted in the planes going 200~220km/h. So these issues also made IL-2 gunnery very difficult, as planes destabilized more during near stall speeds.
However, most of those issues were corrected in the FB add-on. In general management and maneuvering, the feel of the planes became very simular to AH as a whole.
But the minor disturbances and movements during maneuvering remained - they are unnoticeable while just maneuvering, but at the moment where you start to aim, the very very small but continuous movement keeps 'drifting' the target around, and becomes a noticeable factor at the moment of opening up. Machine gun fire is pretty stable(I can hit up to 400 meters or so, even in FB, if I concentrate a lot, spray a lot, and the target flies straight and level), but the vibrations of cannons influence the 'minor movements' a bit more. Larger cannons, from 30mm and upper, have even more distinct effect while shooting.
If I remember correctly, the 'vibration' of the gunfire in AH does not influence the gunnery itself. I think it was a cosmetic feature added in where you feel it only in the cockpit as the 'camera' shakes, to give you an illusion that the platform you are flying, is shaking due to recoil. However, in FB, the shakes, aren't just the screen shaking, mimicking the recoil effect. It comes from the plane itself, which influences your aim if held on for too long.
So, basically, that is my answer to "flying on rails" in AH. I admit, this one is a lot of pure speculation.... so please correct me on this one. More references, experiences, clarifications, please.