Im not much of a Trainer right now, but I can at least show you the basics of ACM and shooting. If nothing else, I am a pretty target.
The key to gunnery is the lead you need to pull. And trust me, if you think you pull enough ahead, double the lead and then you'll hit

.
Theres an easy, albeit slightly boring way to practice your "gun eye" - pick on the drones in offline mode, using the "lead computed gunsight" - it will show you a crosshair where to put your gun pipper to hit at any given time. In offline mode, you will have to turn it on in the Arena Settings first. For that, you open your clipboard -> Options -> Arena Setup-> Enviroment -> Arena Settings.... then you get a list of weird "parameters". Go find FlightModeFlags and check the box for the lead computed gunsight. Hit OK. Now you can "mark" a target with your TAB key and should get the green LCG. On a side note, the LCG works in the TA for friendlies, you just have to enable friendly lock but switching it on with CTRL_TAB, then you can lock targets with TAB same as offline.
Another tip for gunnery. The heavier your bullets, the more they drop, the more lead you need to pull. Start with Cal50 MGs... they do reasonable damage, while being lighter then canon rounds. Also, dont try to shoot canon at the same time as MGs (like on the Spits for example).... due to much different ballistics, they only hit target at the same spot under very special convergence and distance settings. For some advanced thoughts on convergence and gunnery, try
http://trainers.hitechcreations.com/ or
http://www.slowcat.de/slowcats1/AHTC/Convergence.pdf.