Can't you just use 'within' and '45 degrees' instead?
On impact simply compare the angle of bullet and target, eg:
( Bullet.Angle - Target.Angle ) is within 45 degrees of angle 180 = Front
( Bullet.Angle - Target.Angle ) is within 45 degrees of angle 270 = Left
( Bullet.Angle - Target.Angle ) is within 45 degrees of angle 90 = Right
( Bullet.Angle - Target.Angle ) is within 45 degrees of angle 0 = Rear
You might need to switch your left/right on that, but something similar should work.