Zeroing a red dot fits on a postcard: mount it, shoot a group, measure the miss, dial the difference, shoot again. Almost nobody fails at that. They fail around it — nothing lands on paper, the group will not repeat, the turret clicks and the holes stay put. The steps below are the postcard; the section after is what it leaves out, keyed to what you are looking at when it goes wrong.
Before the first shot
Your club or range sets the rules for distance, position and rate of fire, and their procedure comes first. You want a solid rest, a target with one small unambiguous aiming mark, a backer sheet considerably larger than that target, and the fitting tool from the optic box. We sell optics, magnifiers and mounts and nothing else, so targets, bags and bore sighters come from a general shooting supplier.
Check whether you need that tool at all: the Trijicon RMR HD is specified for tool-less zeroing in 1 MOA clicks you can feel and hear, while the Trijicon SRO and Holosun HS407K-X2 ship with one, and that small tool is what gets left on the kitchen table.
The procedure
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Fix the distance first. You will hear 25, 36 and 50 yards (23, 33 and 46 m) named for rifle red dots, and 10 to 25 yards (9 to 23 m) for pistol work. Those are conventions from particular platforms and disciplines, not a standard, and we publish no recommended figure — take the distance from your club, your range's layout or your discipline's practice. The trade-off runs one way: a closer zero is quicker to set and needs less space; a further zero puts the crossing point where you intend to shoot and leaves a larger gap close in.
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Confirm the mounting before you blame the sight. Two joints sit under most optics: the clamp or lever gripping the rail, and the screws running through the sight body into a mount, plate or milled slide cut. Both want the maker's torque figure, and screw length matters as much as tightness. An RMR arrives with long screws for a rail mount, so a milled slide calls for the short screws and sealing plate in the RMR footprint mounting kit. Long screws in a shallow cut bottom out and feel tight while clamping nothing.
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Get on paper before you try to get accurate. Set up at a third of your chosen distance or less, behind a backer twice the width of the target. A 20 cm error at 10 m is on the backer telling you which way to go; at 50 m it is in the stop butt telling you nothing. Where the platform allows it, bore sight first: remove the bolt, look down the bore at a fixed object, and without moving the rifle bring the dot onto that object. That is impossible on a break-barrel air rifle or a fixed-barrel airsoft replica, where close range and a big backer are the whole method.
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Fire a group, never a shot. Three to five, same aiming mark, same hold, same rest. One hole carries your flinch and your hold error and you cannot separate either from the sight. A group's centre is a measurement; one hole is an anecdote.
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Measure from the mark to the centre of the group, then convert. Say the mark is at 25 m (27 yd) and the group's centre sits 5 cm low, 3 cm left. One MOA is about 0.73 cm at 25 m, so that is roughly 7 MOA of elevation error and 4 MOA of windage. On an optic stepping in 1 MOA — the Holosun HS507C-X2 and Trijicon RMR Type 2 both do — 7 clicks up, 4 right. On a 0.5 MOA optic such as the SIG ROMEO5 Gen II or Holosun HS510C, 14 and 8. The arrows describe where the shot moves, not the dot; if the first correction doubles the error, dial back twice what you put in. Full arithmetic and a verified click value for every optic we stock: what MOA means on a red dot sight.
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Tap the turret, then confirm with another group. A firm tap with the tool handle settles an adjustment that has moved but not yet been loaded; without it the next shot or two can land between old setting and new. Read the confirming group, not its first hole.
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Write it down. Distance, projectile, which mount, which rail slot. That note rebuilds the zero after a knock or a year away.
Fault finding, by what you are looking at
Nothing is on paper at all
The instinct is to start turning turrets, which spends a tin of pellets discovering nothing. Halve the distance instead, then halve it again, behind backer wider than any error you can imagine. Where you can see down the bore, bore sighting usually puts you on the backer with the first shot. Where you cannot — most air rifles, most airsoft replicas — shoot at 5 to 10 m (5 to 11 yd) at something large and pale until a hole appears, then walk it out. An optic still missing a big backer that close has a mounting problem, not a sighting one.
The group wanders between shots
A group that opens over a string, or shifts bodily and back again, is mechanical almost every time. Three places produce it, each looking different.
| Joint | What it is | How looseness shows | What settles it |
|---|---|---|---|
| Rail to mount | Clamp, nut or throw lever on the rail | Large abrupt shifts; a mount you can rock by hand | Clamp torque to spec, same slot every time |
| Mount or plate to optic | Screws through the sight body into mount, plate or cut | Group opens steadily, tightens when you retighten | Right-length screws, right plate under, torqued to spec |
| Inside the optic | The windage and elevation mechanism | Small drifting shifts just after adjusting | Tap the turret; judge by the second group |
The middle row is the one people miss: a rail interface can be tested by hand, an optic-to-plate interface cannot. It gains a joint each time you stack hardware — a Holosun EPS on an RMR-cut slide sits on the RMR-to-K adapter plate from the box: two screwed joints in series, not one.
The dot moves but the impact does not
Two causes. The first is running out of travel. Adjustment range is finite and fixed by the mechanism: the HS507C-X2 and HS510C are specified at ±50 MOA, the compact HS407K-X2 at ±30 MOA, the RMR Type 2 at 150 MOA total. At the end of that range a turret keeps turning without moving anything, and on a small-bodied optic you can reach it while still off target.
The second is a mount or plate that consumed the range before you started: an optic on a wedge, a plate not flat, a rail not true. Either way the fix is not on the turret. Count the clicks end to end, set the adjustment near centre, correct the mechanical offset, re-zero. Dozens of clicks with no response is the sight saying it is being asked to rescue the mounting.
Impact shifts after you remove and refit the optic
Quick-detach mounts are common on rifle optics: the HS510C ships on one at 1.41 in lower-1/3 co-witness height, and the EOTech EXPS3-0 carries an integrated quick-detach throw lever on a 7 mm raised base. Return-to-zero is real but narrower than it sounds: the mount repeats its own clamped position closely when refitted the same way. It does not promise every refit is identical, and it says nothing about a different rail slot.
So refit to the same slot, against the same face, lever closed to the same tension — mark the slot if the rail is not indexed — then confirm with a group, because the promise is that the shift is small, not absent. A large repeatable shift usually means lever tension set wrong for that rail, which most adjustable levers have a screw to correct.
Two nearby cases differ. Changing mount height is not a refit but a new zero: swapping a Trijicon RM33 low mount for the taller RM34 moves the sight line relative to the bore. A battery change disturbs nothing where the cell loads with the optic in place, as on the ROMEO5 Gen II's side tray; where it loads from underneath, budget a confirming group. Our battery life and maintenance guide covers the rest.
Zeroed at one distance, wrong at another
Not a fault. Your sight line is straight and the projectile's path is not: it leaves below that line, climbs across it, and falls away. A zero sets where the two cross, so at every other distance there is a gap above or below, sized by how fast and how heavy the projectile is.
The effect is far larger for some disciplines here than others. The Guide on firearms licensing law sets the exempt thresholds at 12 ft lb for an air rifle and 6 ft lb for an air pistol (s.2.46). Airsoft sits under a separate definition: Firearms Act 1968 s.57A sets the limits at 1.3 joules for a weapon discharging two or more missiles successively without repeated trigger pressure and 2.5 joules in any other case, the missile being spherical plastic no more than 8 mm across. A projectile launched inside limits like those follows a far more curved path than a centrefire rifle round, so the gap between two zero distances is practical rather than academic. Scotland runs its own scheme: s.2 of the Air Weapons and Licensing (Scotland) Act 2015 makes it an offence to use, possess, purchase or acquire an air weapon there without an air weapon certificate issued by Police Scotland. Northern Ireland has its own firearms legislation (s.4.5). Confirm what applies to you with your police firearms licensing department, or Police Scotland.
We hold no ballistic data for pellets, BBs or ammunition, and no table here would match your projectile or conditions, so measure rather than look up. With the zero confirmed, shoot a group at each of the other distances you use and write the offset on the same card: a holdover list built from your own kit.
What the zero is attached to
All of it points at one idea: a zero is not a property of the sight. It belongs to a combination — this optic, this mount, this height, this slot, this projectile, this distance — and changing one changes what is being described. If the optic is not fitted yet, the decisions constraining everything above are which pattern it bolts to, in red dot sight footprints explained, and how high it sits, in how to co-witness a red dot with iron sights. Hardware for both is in red dot sight mounts; click values sit on each red dot sights page.