Sighting in a rifle scope is a hardware problem before it is a shooting problem. Get the ring height right so your cheek sits naturally on the stock, set eye relief with the rifle shouldered in your real shooting position, level the reticle to the rifle rather than the horizon, and tighten the rings evenly to the number your ring maker publishes.
Only then does turret math matter, and it is arithmetic: one MOA subtends 1.047 inches at 100 yards, one milliradian subtends 3.6 inches, and both scale with distance. This article assumes you know the general zeroing sequence. If not, start with how to sight in a scope and come back for the hardware and the numbers.
Remove the magazine, lock the action open and physically check the chamber is empty before mounting, leveling, torquing or dialing. Every procedure below happens on an unloaded firearm with the muzzle in a safe direction.
All live fire belongs on a range or a location with a backstop that will positively stop your rounds, with a clear view of what lies beyond. Follow the range’s rules on cold and hot lines and never handle the rifle while anyone is downrange.
Rings, bases and ring height
Ring height determines how comfortable the rifle is to shoot, and most people pick it by eye and regret it. Two constraints pull opposite ways.
Low enough that your head sits naturally. Shoulder the rifle with your eyes closed, then open them. You should be looking straight through the scope. If you have to lift your head to find the picture, the rings are too tall for that stock, and the inconsistent cheek weld produces vertical stringing no turret work fixes.
High enough that nothing touches. The objective bell must clear the barrel and, on a bolt gun, the ocular must clear the bolt handle at full travel. Check by cycling the action with the scope in untightened rings. A folded strip of paper under the objective is the classic test.
Where the stock cannot meet the optic, adjust the stock rather than the rings. On rails, decide fore and aft position before you clamp anything, because that is set by eye relief, not by which slots look tidy.
Eye relief and the eye box
Eye relief is the distance behind the ocular where you get a full, edge-to-edge image. Every scope publishes a figure and a small window either side of it. Set it wrong and you get a black ring around the picture, or worse, the ocular meets your eyebrow under recoil.
- Confirm the rifle is unloaded
Magazine out, action open, chamber physically checked. You are about to shoulder it repeatedly with your hands off the controls.
- Set the rings loose enough to slide
Snug the scope so it holds position but still slides fore and aft by hand and rotates under light pressure.
- Shoulder the rifle in your primary position
Eye relief changes with position because your head moves along the stock. Set it for the position you use most, then check the others.
- Slide the scope until the picture is full on the first look
Close your eyes, mount the rifle, then open them. Adjust until the image is complete without you hunting for it. On anything that recoils meaningfully, bias the scope slightly forward, because the head moves toward the ocular under recoil.
- Focus the ocular on the reticle, not the target
Point at blank sky, glance in for a second, adjust until the reticle is instantly sharp. Stare longer and your eye accommodates, which is how people end up with a permanently soft reticle.
Leveling the reticle to the rifle
A canted reticle does not stop you zeroing. It costs you at distance, because the elevation dial no longer moves the bullet straight up relative to gravity. The further you dial, the further to one side the correction goes.
The reference that matters is the rifle, not the bench and not the horizon. Two approaches work:
- Level the rifle, then the scope. Level off a flat machined surface of the receiver or a leveled rail, get the rifle plumb, then level off the flat of the elevation turret cap and rotate the scope until it agrees.
- Plumb line. Hang a weighted string at distance, secure the rifle, rotate the scope until the vertical stadia sits on the string. Simple, free and surprisingly accurate.
At normal hunting distances a small cant is unlikely to be why you missed. It becomes measurable once you dial significant elevation, because a tilted erector moves the bullet along a tilted axis. If you dial for distance, use a plumb line and consider an anti-cant indicator.
Torque, evenly and in sequence
Two failure modes, and they are opposites. Under-tightened rings let the scope creep under recoil, which reads as a scope that will not hold zero. Over-tightened rings pinch the tube, which can bind the erector and cause tracking problems that look like a defective optic.
Use the value your ring and base manufacturer publishes. Ring cap screws, ring-to-base clamps and base-to-receiver screws are usually three different numbers and vary widely by brand. Do not carry a figure over from another set of rings, and do not take one off a forum. The spec sheet in the box is the source. The technique matters more than the number:
- Tighten in a cross pattern, a little at a time, so the cap seats evenly.
- Aim for an even gap on both sides of the ring cap where the design calls for one. A cap bottomed out on one side is clamping unevenly.
- Use an inch-pound torque driver. Ring screws live in a range where hand feel is unreliable and a foot-pound wrench cannot resolve it.
Recheck the mount after the first range session and after hard travel. Most zero-shift complaints resolve here.
MOA and MIL: what a click actually moves
Both systems are angles, and an angle covers more area the further out you go. That fact explains every correction you will make.
- 1 MOA at 100 yd
- 1.047 in
- 1 mil at 100 yd
- 3.6 in
- 1/4 MOA click at 100 yd
- 0.26 in
- 0.1 mil click at 100 yd
- 0.36 in
MOA is minute of angle, one sixtieth of a degree. At 100 yards it subtends 1.047 inches, which is why plenty of people call it an inch and get away with it. That approximation is off by about half an inch at 1,000 yards: irrelevant inside a few hundred, worth correcting past that.
A milliradian is one thousandth of the distance to the target: 3.6 inches at 100 yards, or exactly 10 centimeters at 100 meters. A tenth-mil click, the common increment, moves the bullet 0.36 inches at 100 yards.
The consequence: a correction at 25 yards costs roughly four times as many clicks as the same correction at 100. On a quarter MOA scope, one inch of movement is about 15 clicks at 25 yards, 8 at 50, 4 at 100 and 2 at 200. That is why close-range adjustment feels like it is doing nothing, and why people over-correct when they move back.
Worked corrections you can copy
Work from the center of a three shot group, never a single hole, and correct one axis at a time.
Quarter MOA scope, 100 yards, group 3 inches low and 2 inches right. Treat 1 MOA as an inch here. Three inches is 3 MOA, or 12 clicks up. Two inches of windage is 2 MOA, or 8 clicks left. At 50 yards the same scope needs double the clicks, because a click moves half as far.
Tenth mil scope, 100 yards, group 3 inches low and 1.5 inches left. Divide inches by 3.6 to get mils. Three inches is 0.83 mil, or 8 clicks up. One and a half inches is 0.42 mil, or 4 clicks right.
Mil scopes get easier if you stop converting. With mil hash marks in the reticle, measure the miss in the reticle instead of in inches. A group two hash marks low on a tenth-mil turret is 0.2 mil, or 20 clicks, at any distance and with no arithmetic. That is the strongest argument for mil over MOA.
A mil reticle with an MOA turret, or the reverse, forces a conversion under time pressure and is a well-known source of errors. Whichever system you choose, make sure the reticle and turrets both speak it. If you inherit a mismatched scope, tape a conversion card to the stock.
Parallax looks like a zero problem
Parallax is what happens when the target image and the reticle are not on the same focal plane. Move your head behind the scope and the reticle appears to swim across the target. Every shot taken with your head in a different place lands in a different place, and the result is a group that opens up for no visible reason.
Fixed-parallax scopes are set at the factory for one distance, commonly around 100 yards on centerfire optics. Adjustable objective and side-focus scopes let you dial it out: move your head slightly while looking through the scope and adjust until the reticle stops shifting against the target. Do not set it by whichever position makes the image look prettiest, and do not trust the yardage markings on the ring, which are frequently approximate. Re-set it whenever you change distance.
Turret types and zero stops
The turret you have shapes how you use the zero.
Capped, set and forget
- Nothing to snag on gear, brush or a sling
- Cannot be knocked off zero in a pack or a case
- You hold over for distance rather than dialing
Exposed, dial to distance
- Fast, repeatable elevation without holdover
- Needs a zero stop or a careful count to get home
- Rewards a written dope card for your load
Once the zero is confirmed, reset the dials to read zero if your scope allows it. The mechanism varies, so follow the manual: doing it wrong on some designs moves your zero rather than just the numbers on the dial.
One hardware detail people miss after the optic is sorted: how the rifle is supported can shift point of impact. Tension through a forward sling attachment that loads the barrel, rather than the handguard or a free-floated section, can move where the rifle prints, and so can a bipod loaded against a barrel-mounted stud. If your rifle has a stud in the barrel channel, confirm your zero with and without sling tension. Our guide to the two point sling for AR15 covers where to route webbing, and the best AR slings comparison covers attachment options.
Frequently asked questions
What is the difference between MOA and MIL?
Two ways of measuring the same thing. One MOA subtends 1.047 inches at 100 yards; one milliradian subtends 3.6 inches, or exactly 10 centimeters at 100 meters. Neither is more accurate. Choose the one your reticle speaks, and make sure the turrets match.
How many clicks move one inch at 100 yards?
About 4 on a quarter MOA scope and about 3 on a tenth mil scope, since a tenth mil click moves 0.36 inches. Halve those at 200 yards, double them at 50.
Do I really need a torque wrench for scope rings?
If you mount your own optics more than once, yes. The gap between too loose and tube-crushing is smaller than most people expect. Use the value your ring manufacturer publishes rather than a general number.
My scope tracks one direction but not the other. Is it broken?
Test it first. Shoot a box test: from a confirmed zero, dial a known amount up, fire a group, then right, then down, then left by the same amount, and see whether the group returns to the start. If it does not and the mount is tight, the optic is the suspect.
