Dieg777 has submitted a gunnery tutorial. It consists of a written guide, links to useful sites and uniquely two sets of missions for gunnery practice.
Please try out these missions and let us know what you think.
Read the full Snapshot Guide:
http://www.airwarfare.com/guides/snapshot.htm
edit- just to highlight that these are for a merged install as they use the spitfire and FW190 dora for examples.