How to Test Whether Your Scope Tracks Properly: The Tall Target Test, UK Version

First, know what your turret should do. A scope's click value is an angle, so the movement on the target grows with distance. For mils, 1 mil is 10 cm at 100 m, so a 0.1 mil click is 1 cm at 100 m, 0.5 cm at 50 m and 0.25 cm at 25 m. For MOA, 1 MOA is about 1.047 inches at 100 yards, which is 2.91 cm at 100 m, 1.45 cm at 50 m and 0.73 cm at 25 m. A quarter-MOA click is therefore about 0.73 cm at 100 m, or about a quarter of an inch at 100 yards. Check your scope's manual for how the click value is quoted: some makers give it at 100 yards, some at 100 metres, and the two differ by about 9 per cent in size.

Second, choose your distance. At a range with 100 metres or 100 yards, use it, because clicks are large enough to measure. Convert the numbers for the distance you actually have. For an air rifle, 25 metres or 25 to 30 yards is a sensible test distance. At 25 m a quarter-MOA click is under 2 mm, too small to measure, so do not test one click at a time. Dial a large amount, such as 5 mil, which is 12.5 cm at 25 m, or 20 MOA, which is about 14.5 cm at 25 m. The bigger the dialled movement compared with your group size, the more accurately you can measure the tracking.

Third, build the target. You need a vertical line and a way to measure up it: a strip of paper or cardboard, about 1 metre tall or as tall as your target frame allows, with a bold aiming point at the bottom and a ruled scale up the side. Hang it straight using a plumb line or spirit level. The vertical line also shows sideways drift. At 100 m, a metre of height covers 10 mil or about 34 MOA, which is plenty. Check with the range officer what you may fix to the butts.

Fourth, set the rifle up. A scope can only be tested as well as the rifle holds still. Check that rings, bases and the rail are tight, and that the reticle is level in the scope with the rifle level. Use a good rest and the same pellet or load throughout. Set the parallax or focus to the test distance. Cant is the main trap: a rifle leaning a few degrees sends the groups sideways and makes the vertical movement slightly shorter than it should be. Level the rifle on a level surface before each group.

Fifth, shoot the test. Shoot a three-shot group with a centrefire or rimfire rifle, or five shots with an air rifle, at the bottom aiming point and mark the centre of the group. Then dial up a known amount, say 4 mil on a centrefire rifle. Aim at the same bottom point again and shoot another group. Continue in steps, up to the largest amount you will ever use or as far as the target is tall. Then come back down in the same steps and shoot a final group with the turret back at its starting position. Do not wind the turret hard against its end stop at any point. If you can feel it bottom out, stop and back off.

Sixth, measure and compare. Measure from the centre of each group to the centre of the first, along the vertical line, and compare with the expected movement. Example 1, a mil scope at 100 m: you dial 10.0 mil (100 clicks of 0.1 mil) and expect 100 cm. You measure 98 cm, so the scope moved 2 per cent less than its markings. A 3 mil correction would then arrive 0.06 mil short, which is 0.6 cm at 100 m and 1.2 cm at 200 m. Example 2, a quarter-MOA scope at 100 yards: you dial 20 MOA (80 clicks) and expect 20 x 1.047 = 20.94 inches. You measure 20.5 inches, about 2 per cent short. Example 3, an air rifle at 25 m with a mil scope: you dial 5.0 mil and expect 12.5 cm. You measure 12.0 cm, 4 per cent short. But if your five-shot groups are 1 cm across, the centre of each is uncertain by a few millimetres, which is a few per cent of 12.5 cm, so repeat with a larger dial amount before you decide anything.

Seventh, read the return. After the climb, the final group at the original setting should land back on the first group. If it lands lower or higher, the turret is not returning to the same place, and that matters more than a small difference in click value, because you can allow for a consistent 2 per cent error in your dope but you cannot allow for a random one. Also check the sideways position. All groups should line up on the vertical line. If they drift consistently to one side as you climb, the reticle may be canted against the turret's line of travel, or the rifle was canted at the shot. Repeat with extra care over levelling before you blame the scope.

A no-ammunition version. Clamp the rifle in a vice or hold it very steady on a rest, put a ruled target at a known distance, set parallax to that distance, dial a known amount and see where the crosshair now sits against the ruler. For an air rifle this can be done in a safe indoor space with the rifle empty and uncharged. It checks the turret's travel, but it cannot show whether the scope holds zero when fired, and your reading of the reticle against the ruler has its own error, so it suits larger movements.

What to do with the result. Decide beforehand how much error matters. If your longest realistic dial is 2 mil, a 2 per cent error is 0.04 mil, which is 4 mm at 100 m and well inside your group. If you dial 12 mil in competition, 2 per cent is 0.24 mil, which is 24 cm at 1,000 m and could matter. If the scope tracks consistently, write the measured value on your dope card and use it. If it is erratic, if some clicks do not register, or if it does not come back to zero, take your target, photographs and notes to the retailer or importer. A clear record beats a general complaint.

Reticles and magnification. The test checks the turret, not the reticle. If you use the reticle for holdovers, check it separately by shooting at two aiming points a known distance apart. In a second focal plane scope the subtensions are correct at only one magnification, which the manual states, so test at that setting. In a first focal plane scope they hold at every magnification.