1. the engine are not immune... you probably not touch the motor at the right place to make real dammage
2. that can happen... if you don't touch an vital part of the plane...this one will go on to fly with no real dammage. in fact it is realistic because 7,7mm bullet on hurri or spit are not good at all to make dommage... english will understand this later and change for 12,7mm with better performance. during the bob... most of time... Two or three fighter were necessary to destroy a bomber. i suggest you to make front attack and shoot the cockpit or attack by up or down side... the shoot at just 6 O'clock of a plane and at same level are less effective.
3. The gunners are really bad and don't act quickly enough...IA must be fixed.
4. german using 9,72 who is more effective than 7,7. and by the way... they shoot directly your engine because you are rear the plane... your bullet most of time touch the wings of a plane and not his engine.
5. sometimes you can have the luck to shoot just in a vital area. just in the fixation of wings...or sometimes just in the fuel and it explose. that is not a bug...it is just a question of aiming and shooting at the right place with the right munition.