Two Straight Lines That Are Not the Same
Interpreting any trajectory chart depends on distinguishing two straight reference lines. The line of sight runs from the eye through the aiming system to the target, while the bore line, also called the bore axis, runs straight out along the centerline of the barrel. Confusing them makes the shape of a trajectory impossible to explain correctly.
The Sight-Over-Bore Height
The aiming system, whether iron sights or an optic, sits physically above the barrel by a fixed distance often called sight height or scope height. Because the eye looks along a line above the bore, the two lines start apart at the muzzle. This vertical offset, commonly an inch to a couple of inches, sets up the geometry of the entire trajectory.
The Small Launch Angle
Since the projectile falls under gravity the instant it leaves the muzzle, the barrel must be tilted slightly upward relative to the line of sight so the arc can climb up to meet the aiming line. This deliberate upward tilt is the angle between the bore line and the line of sight, and it is what a shooter sets when zeroing.
The Two Crossings
Because the projectile starts below the line of sight, rises across it, arcs over, and falls back through it, the trajectory intersects the line of sight at two distances. The nearer crossing occurs early as the projectile climbs, and the farther crossing is the distance most people call the zero. Between them the path rides above the sight line.
Why This Explains Trajectory Shape
These two lines together explain the familiar sequence of a projectile starting low, rising to and above the line of sight, then dropping below it far downrange. The bore line points where the barrel is aimed; the line of sight points where the eye is aimed; and the gap between them, opened by sight height and closed by the launch angle, is the reason zeroing exists.
Worked Example
Picture an optic mounted 1.5 inches above the bore, zeroed at 100 yards. At the muzzle the projectile starts 1.5 inches below the line of sight. It crosses the line of sight the first time around 25 yards, rides slightly above it through the middle distances, and crosses again at 100 yards, the far zero. Beyond that, gravity pulls the path steadily below the line of sight.
A Common Misconception
A frequent error is imagining the projectile leaves the barrel already aligned with the line of sight and simply falls from there. In truth the bore is angled upward beneath the sight line, so the projectile first travels below the line of sight and climbs to meet it. Ignoring the sight-over-bore offset makes the near crossing and the initial rise inexplicable.