Guides · Field practice

Scope Tracking: The Tall Target Test and What a 3% Error Costs

How to run a tall target test, turn it into a tracking factor and correct your come-ups, what a turret that moves 3% too far does at 1,000 yards, and why truing velocity can hide it.

6 min readSolved with the Ballista engineUpdated October 2026

A scope that moves 3% more than its marks sounds close enough. Dial the 6.5 Creedmoor’s 8.85 mil come-up for 1,000 yards on it and the turret really moves 9.11 mil. The shot lands 9.6 inches high, 2.7 clicks off, before wind or anything else. At 300 yd the same turret costs 0.4 in and you’d never see it. The error is a fixed percentage of whatever you dial, so it hides close in and shows up exactly where you’re least sure of everything else.

The tracking factor

The tracking factor is what the turret really moves divided by what its marks say:

tracking factor = measured movement ÷ dialed movement

A factor of 1.03 means 3% too far, 0.97 means 3% too short. Once you know it, the correction is one division: dial the solver’s come-up divided by the factor. On a 1.03 turret, a 10.00 mil come-up becomes 9.71 mil. Since one 0.1 mil click is 1% of 10 mil, a 3% error is one click for every 3.3 mil you dial.

There’s one common case where the factor is known in advance. Some “1/4 MOA” turrets move a quarter inch per 100 yd rather than a quarter of a true minute of angle. A true MOA is 1.047 in at 100 yd, so those turrets have a factor of 1 ÷ 1.047 = 0.955 against true MOA, a 4.5% shortfall.

What 3% costs

On a 12 in plate you have 6 in from the center to the edge. A 3% error uses 2.5 in of that at 600 yd for the 6.5 and 3.1 in for the .308, before wind, velocity spread or the shooter. At 1,000 yd it misses the plate on its own: 9.6 in high for the 6.5 and 12.9 in for the .308.

The tall target test, step by step

1. Measure the distance with a tape, to the face of the target. The expected movement scales with distance: 1 yd off at 100 yd is a 1% error in the factor, a third of what you’re trying to measure.
2. Hang a target with a vertical line on a plumb bob or check it with a level. You need true vertical to see whether the turret moves straight up.
3. Level the rifle on a solid rest and fire a group at an aiming mark near the bottom.
4. Dial up a large, round amount without moving the rifle’s aim: 10 mil, or 30 MOA, or as much as you use at your longest distance. Hold the same aiming mark and fire a second group.
5. Measure center to center. 10 mil at exactly 100 yd should move 36.0 in. 30 true MOA should move 31.4 in, and 30 inch-per-100-yd clicks move 30.0 in.
6. Dial back down and fire a third group. It should land on the first. If it doesn’t, the turret isn’t returning to zero, which is a bigger problem than tracking.
7. Look at the line. If the second group sits left or right of the plumb line, the turret’s up isn’t the rifle’s up, or the rifle was canted. Either way it’ll show up as a sideways miss at distance.

Use enough elevation that the movement is big compared with your groups. With a rifle that averages 1 MOA for 5 shots, each shot’s vertical standard deviation is about 0.33 MOA, so a 3-shot group’s center is uncertain by 0.19 MOA and the difference between two centers by 0.27 MOA. Over 30 MOA of travel that’s a 0.9% uncertainty in the factor. Over 10 MOA it would be 2.7%. Dial big, shoot 3 to 5 shots per group, and repeat the test if the answer is close to the error you care about.

Velocity truing hides a tracking error almost perfectly

If you don’t know about the turret, the misses at distance look like a velocity problem. A turret that moves too far puts shots high, which looks like a bullet that’s faster than the solver thinks. True velocity at 600 yd with the 6.5 and the engine settles on 2,745 fps instead of 2,710.

You might expect the false velocity to show itself farther out or in different air. It barely does. While the bullet is supersonic, a faster bullet needs less elevation by close to the same percentage at every distance, which is exactly what a turret error looks like. The trued solution stays within 0.004 mil from 300 to 1,000 yd, and within 0.03 mil out to 1,400 yd, where the bullet is subsonic. At a 6,000 ft density altitude it’s no worse. The only trace in this example is the wind: the solver thinks the bullet is faster, so it holds 0.039 mil too little for a 10 mph crosswind at 1,000 yd.

So elevation alone won’t tell you. What gives it away is a velocity that disagrees with your chronograph by more than the chronograph’s error, here 35 fps, or a come-up that’s wrong on a different rifle or load under the same scope. A tall target test before truing keeps the turret’s error out of your velocity, so the trued number means what it says.

Ballista’s truing sheet treats the turret as one of the possible causes of a miss. When the turret could explain what you saw, the list of other causes shows “Turret” with how far off it would have to be, in percent, and “A tall-target test settles it.” Its warning for a large velocity change also names scope tracking: “A wrong distance, click value or scope tracking is more likely than the bullet.” For quarter-inch turrets, the scope’s click setting includes “1/4 in @ 100 yd”.

Routine

1. Run a tall target test when you mount a scope, with the travel you actually use.
2. If the factor is off by more than about 1%, divide your come-ups by it, or have the scope serviced.
3. Test before you true velocity or BC, so truing fixes the bullet and not the turret.
4. Re-test after a hard knock, and check the return to zero every time you shoot at distance.