I use 200 for both cannon and mg's, I like my shots concentrated, so that one small burst with both, take down the opponent, as for range, let them ripp at .30, .20, and for the p39, and p63, I have gotten quite accurate with 1 shot, 1 kill mindset, my aim is just high, when behind fighters, and side passes on bomber, 1 shot will usually cut the tail off, aiming for say, top gunner, that would be your lead for a tailshot, I practiced in quick mission bulder with flights of bombers to nail down that p-39's big gun. It's my favorite plane for firepower, against bombers.