While I think offline gunnery missions with drones would be a nice convenience, you can make do without them provided you have a practice partner. Here is how a friend and I do it:
We join the same country and move to a remote airbase. We take off, get some altitude and then take turns being drones for each other. Since friendly fire does not (currently at least) damage planes, we live fire at each other. The nice thing about this is that you will see the flashes of your rounds striking your partner's aircraft just as you would on an enemy, so knowing when you are hitting them isn't a problem. We change the rules some, working in other air combat skills beyond gunnery.
If anyone posting here would like to get together for some practice, drop me a note. I could certainly use it. Currently, the maneuver I am practicing most often in arena combat is ditching;p
Lance
FDB2