The rudders are still effective when in a gunner position, thus maybe that's how you are turning.
A suggestion for practicing calibration, you don't actually have to drop a bomb to see how well you calibrated offline. Go offline, choose "setup", "arena setup", "environment", "arena settings" and select "flightmodeflags". Press Change and then turn on the "divebomb sight". That is the little green + sign you see on the ground and it works like a constant computing bomb-impact point. Then, fly your bomber, and calibrate. If you are doing it correctly then the green + and the + from your bombsight crosshairs should line up after you've calibrated. You can practice over and over this way, just seeing how close the two match up, until you get the hang of it. This way, you can calibrate a dozen times in a row and in a single evening probably have the hang of it.
One other tip, you can use the wind to improve your climb-rate (so you need not wait a long time to climb to a practice altitude). Climb in your aircraft, and take-off, level out and get to about 200mph. Then, open up the settings, look for the "Wind" (again under arena settings) and set the "vertical" wind component to max (127)... that puts a 127mph updraft under you and zips you right up to altitiude. Then, press "no Wind" to turn it off so you can get to speed and practice.
A couple of caveats, the green + will show the impact point so undulating terrain at a different altitude than where you calibrated (selected your target) will not match up. Easiest way to ensure it matches is to select something at sea-level along a shoreline and then calibrate near shore. Secondly, you can't use your normal "level auto-pilot" if you are using the wind trick, you need to use "Auto-level" (SHIFT-X) as that will not try and maintain constant altitiude. normal autopilot will try and dive to compensate for the updraft.
Hope that helps.
