Zero it. Then prove it.

A zero is a single point of agreement between the bore and the sight line. Verification is proof that the mechanism between them behaves predictably when you move it. They are different things, and the second one is where scopes get caught out.

Before you fire anything

Mount quality determines zero retention more often than scope quality does. Get this stage right and most "my scope won't hold zero" problems never happen.

• Degrease the mating surfaces. Receiver rail, base underside, ring interiors. Factory preservative oil on a mounting surface is a slip plane.
• Torque to the manufacturer's spec with an inch-pound driver. Base screws and ring cap screws have different specs — commonly in the region of 25–65 in-lb for bases and 15–18 in-lb for ring caps, but use the numbers your specific hardware ships with, not these. Guessing crushes tubes or lets bases walk.
• Tighten ring caps in a cross pattern, in stages, keeping the gap on both sides even.
• Level the reticle to the rifle, not to the world. A canted reticle introduces horizontal error that grows with every click of elevation you dial. Use a level on the action's flat and a second on the turret cap.
• Set eye relief in your actual shooting position, wearing what you'll wear, at the magnification you'll use most. Do it before final torque.

Bore sighting

On a bolt gun, remove the bolt, put the rifle in a stable rest, and look through the bore at a target 50–100 yards out. Without moving the rifle, adjust the scope until the reticle sits on the same spot the bore is pointing at. This costs nothing and usually gets your first live round on paper.

For an AR or anything you can't see through, a laser bore sighter or a collimator does the same job. Neither is a zero — both just save ammunition.

The zero itself

100 yards is the sensible default for a centrefire rifle. It's long enough to be meaningful, short enough that wind is negligible, and it's the distance every ballistic solver expects as an input.

1. Build a genuinely stable position. Bipod or bags front and rear. Your zero can only be as good as the position you established it from.
2. Fire a three-shot group. Let the barrel cool if it's a thin profile.
3. Measure from the group's centre to the point of aim — not from the nearest hole, and not from the flier you want to disown.
4. Convert that distance to clicks. If you're 2 inches low and 1 inch right at 100 yards with quarter-MOA turrets, that's roughly 8 clicks up and 4 clicks left.
5. Dial, fire another three, confirm.
6. Reset the turrets to read zero, following your scope's procedure — usually loosening set screws and repositioning the dial without moving the erector.

The box test: does it return to zero?

This is the test that catches the failure that actually strands people — a scope that dials out fine but doesn't come back.

1. Fire a group at your aiming point. Call this corner one.
2. Dial a fixed amount up — say 4 mils or 12 MOA. Fire a group, aiming at the same original point.
3. Dial the same amount right. Fire a group.
4. Dial the same amount down. Fire a group.
5. Dial the same amount left. You are now back at the original setting. Fire a final group.

You should see four groups at the corners of a square and the fifth group superimposed on the first. Any of the following means the mechanism is not trustworthy:

• The final group has walked away from the first — the erector isn't returning.
• The box isn't square — the vertical and horizontal adjustments aren't perpendicular, or the reticle is canted.
• The sides aren't equal lengths — click values differ between axes.

The tall target test: what is a click really worth?

The box test proves the scope returns. The tall target test measures the actual value of each click, which is the number your ballistic solver depends on.

1. Hang a target with a verified plumb vertical line at least 40 inches tall. Use a plumb bob or a level — a line that is off vertical corrupts the whole test.
2. Set the target at a laser-measured distance. Not paced, not estimated. A 2% distance error becomes a 2% click-value error.
3. Aim at a mark at the bottom of the line and fire a group. Do not move your aiming point again for the rest of the test.
4. Dial a large, known elevation change — enough to move roughly 30 inches at your distance. More travel means smaller measurement error.
5. Fire another group at the same aiming point.
6. Measure the vertical distance between the two group centres, precisely.

Now compare measured against expected. Say you dialled 10 mils at a laser-verified 100 yards. Expected movement is 36 inches. If you measure 34.2 inches, your scope is tracking at 95% of its claimed value.

That 5% is invisible at 300 yards and puts you well off a target at 1,000. The fix isn't to send the scope back — it's to enter the correction factor into your ballistic solver, which every serious app supports. Now your dope is right for the scope you actually own.

Horizontal deviation from the plumb line during the test also reveals reticle cant. If the impacts drift sideways as you dial up, the reticle isn't level with the rifle.

What to do when it fails

Before blaming the optic, work through the cheap causes in order: loose ring caps, loose base screws, a loose action in the stock, a barrel contacting the forend, a bipod loading the rifle differently shot to shot, or a parallax setting that lets your head position move the reticle. Parallax specifically masquerades as poor precision — set it so the reticle stops floating against the target when you shift your head, at the actual distance you're shooting.

If all of that is clean and the box test still fails, the scope is the problem. Most reputable manufacturers will service or replace it.

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