slip and ball ...

[nearmiss]

The slot and ball instrument is the Slip Indicator. This instrument shows if the aircraft is in coordinated flight or not. When an aircraft is "coordinated", the nose is aligned with the aircraft flight path. When the ball is outside the turn (on the opposite side from the direction of turn), the aircraft is said to be in a skid. When the ball is inside the turn (on the same side of the turn), the plane is said to be in a slip. In either case, the plane may be brought into coordinated flight by "stepping on the ball", that is, applying rudder in the direction that the ball is until it is centered again. In general, an aircraft turns most efficiently when it is in coordinated flight, but there are tactical reasons why a pilot may wish to fly uncoordinated.

As far as targeting enemy aircraft remember your guns have barrels and these barrels are aligned within the aircraft. You can look through your gun site at the enemy and center him in the gunsite, but that doesn't mean your guns are aligned as your aircraft can be in a skid and the guns will not shoot as the gunsight shows, but in the direction where the aircraft is aligned.