If you take off from a TA arena with a lot of fighters hanging around normally one or two will come at you, to practice your gunning, if you ask. If not you can put up your own target to practice on. Type . target 600 if you want to practice from 600 out, or, type in any distance.
Gunning from Ostis or fixed gun positions will help you just give it time. And nothing beats spending time in bombers to learn the guns. Ive only been in the game a few months but Ive learned the gun positions in about 4 or 5 bombers so well I dont even have to think the change positions now.
There are an awful lot of tricks to be learned to be a better bomber gunner so be patient. You can be very effective gunning and flying the aircraft with your rudder pedals.
Anytime you want to practice in the TA with me then let me know and we can take turns taking fighters up against the other.
The real issue with bomber guns is everything changes when the angle of attack changes. The leads, the closing speeds...ect
Ive doubles the amount of fighters I shoot down with every month Im in the game. So with practice you will improve. The biggest thing that helped me improve, other then practice, was to fly only a few different bombers.