By B.B. Pelletier
Where to zero a scope is a question that always starts a friendly conversation among airgunners. I’ll tell what your options are and leave the final choice to you.
Pellets start to fall the minute they leave the muzzle
The moment a pellet (or bullet) leaves the muzzle, it begins falling toward the ground. It falls at the same rate it would if you dropped it from the height of the bore – assuming the bore is parallel to the ground! And that’s where the scope adjustment comes in.
Because a scope looks straight out and because it is mounted above the bore, it can only be made to intersect the trajectory of the pellet if it points down through the ballistic path the pellet takes. And, that is how scopes are sighted-in.
They are adjusted to look straight through the downward arcing trajectory of the pellet at a point close to the muzzle. When the pellet arrives at the spot where the downward-looking scope is pointing, the crosshairs will be exactly where the pellet is and the scope will be zeroed at that distance.
Beyond this point, the scope will actually be looking UNDER THE PELLET’S FLIGHT for a certain distance after the first point of intersection. Then the falling pellet will cross back through the scope’s line of sight once more, intersecting the pellet a second time and creating a second zero point.
How we correct the picture!
Nobody likes to think about their pellet falling, so we elevate the barrel so the pellet is actually going slightly up when it leaves the gun. Now, the whole thing makes more sense.
The pellet SEEMS to be rising when, in fact, it is only doing so because the barrel is tilted slightly up at the muzzle. The downward-looking scope intersects the pellet at some distance downrange, then the pellet SEEMS TO RISE above the straight line of the scope and intersect a second time further downrange.
What’s a good distance to sight-in a scope?
Now that we understand how it works, we need to find the right distance to sight-in. You now understand that the scope will actually be zeroed for TWO DISTANCES instead of one.
For a pellet gun that shoots around 800 f.p.s., I like to sight in at 20 yards for the near distance. The second distance will be around 30 yards, and the pellet will not rise by as much as one pellet diameter at the in-between distances (between 20 and 30 yards). If you sight in at 15 yards with the same gun, the pellet will be back to the intersection of the crosshairs around 40 yards, and it will rise more than an inch in between.
For a gun that shoots 950 f.p.s., I would still sight-in at 20 yards as the near distance but the far distance is now 37 yards or so. For both guns (800 and 950), the pellet will be about one inch below the aim point at 10 yards and will rise to the crosshairs as it approaches 20 yards. At the muzzle, the pellet will be as far below the crosshairs as the bore is below the optical path of the scope, which could be as much as three inches or more!
What if I only want ONE sight-in distance?
It IS POSSIBLE to sight a scope to intersect the pellet only once, but why would you want to? You align the scope to graze the pellet’s trajectory, and after that it’s all down hill!