The principle is that the straight line determined by the ruler is basically a trajectory, and the target can shoot on this baseline, thus forming a three-point line.

But the actual shooting didn't really aim at three points and one line. This is because the ruler can correct the drop of bullet height at a certain distance, but it can't correct the crosswind and the movement of the target. Therefore, the design should make corresponding position correction according to the movement of the target. For an experienced shooter, you can eliminate the influence of crosswind accurately without a ruler. So the actual shooting time is not three points and one line, but the left and right correction!