Bullet dispersion is different depending on where the guns are mounted. Wing mounted guns have a higher dispersion than fuselage mounted guns.
Bullet drop is a pure physics calculation.
Also keep in mind, convergence sets the guns in both the X and Y planes. If you set the convergence to 600, the guns are elevated to hit a target dead center at 600. Try moving the target to 300 with the convergence at 600 and watch.