Well now I know whats wrong with your bullets, your not hitting anything, thats why they dont do any damage.
You were firing at targets over 1000 away in a P39, your "lead" was what you should use for a shot at 300-350, so ya, not hitting anything. First off, no matter how bad you want to, dont bother shooting at anything over 600 out. Especially on a moving target. If a guy is AFK and flying straight and level you can pick him apart at 600 out but it will take double the hits because the range just sucks the power out of the bullits. Most shots that land good damage are under 350. Cannon rounds have a much bigger drop because they are heavier. So not only do you need "Lead" you need elevation.
Field guns, the rounds are so slow that you have to lead a target moving from left-right across the screen almost the whole screen! Also at the ranges you were shooting at...over 2k you weren't going to hit him anyway. 1500 is max and even then you better send the shots off with a prayer. 1200 and under are better ranges and the lead is about twice what you would do in a plane, those rounds are just too slow.
You screen looks pretty squashed down too. Im not sure if it is a "recording" problem or it is how you have it setup. Moving your head position back may help.