To mount and zero a red dot: fit it at the right height on a clean rail, tighten the screws to the maker's torque figure, bore-sight it, then fire three- to five-shot groups at a measured distance and adjust until the group centres on your aim point. For a rifle, start at 25 m and finish at 50 m; for a pistol optic, 10–25 m is usual. The whole job takes an afternoon, and nearly every lost zero traces back to a skipped step in the mounting stage rather than the zeroing.
Key takeaways
- Use the torque figure in your optic's manual; SIG, for example, gives about 30 in-lb (3.4 N·m) for the ROMEO5's mount nut.
- Most rifle dots adjust in 0.5 MOA clicks and many pistol dots in 1 MOA clicks, so check before you count clicks.
- 0.5 MOA is about 3.6 mm at 25 m, 7.3 mm at 50 m and 14.5 mm at 100 m.
- A 50 m zero keeps a .223/5.56 within about 5 cm (2 in) of the dot out to roughly 200 m.
- Record whether you zeroed in metres or yards; 50 m and 50 yd are 4.3 m apart and give different far zeros.
What you need
- The red dot, plus the correct mount for a rifle or the correct footprint or adapter plate for a pistol.
- A torque driver that reads in inch-pounds (in-lb) or newton-metres (N·m), with the right Torx or hex bits.
- Medium-strength (blue) thread locker, where the optic maker allows it.
- Degreaser and cotton buds, a stable rest, targets with a 25 mm grid, and ammunition you intend to keep using.
Step 1: choose the mount and height
On an AR-pattern rifle the usual heights are absolute co-witness at 1.41 in (35.8 mm), lower 1/3 at about 1.535–1.63 in (39–41.4 mm), and taller 1.93 in (49.0 mm) mounts for a heads-up position. Many rifle dots include a mount: the Holosun HS510C has an integrated 1.41 in quick-detach mount, for example. Compare every height in our mount height chart.
On a pistol, the optic must match the slide's footprint or an adapter plate. The footprint compatibility chart and our RMR vs RMSc guide explain which fits which. For optics-ready slides that take RMR-pattern sights, Trijicon makes an RMR footprint mounting kit for several common slides.
Step 2: mount the optic
- Degrease the rail or slide cut and the screw threads. Thread locker will not grip oily threads.
- Seat the mount in the rail slot, then push it forward against the recoil direction before tightening, so recoil cannot shift it.
- Apply a small drop of thread locker to each optic screw if the manual allows it, and start all screws by hand.
- Tighten gradually, alternating between screws, to the torque in your manual. SIG's ROMEO5 manual gives "hand tight plus ¼ turn or approximately 30 in-lbs" for its mount nut. Pistol optic screws are much smaller and use far lower figures, so never borrow a rifle number.
Over-tightening strips threads and can crack housings. Correct torque, not brute force, is what holds zero.
Step 3: let the thread locker cure
Give the thread locker time before you shoot. Henkel's performance data for Loctite 243, a common medium-strength thread locker, is quoted after a 24-hour cure at 23°C, so leaving the rifle overnight in a warm room is sensible. Cold garages slow the cure.
Step 4: bore-sight
Bore-sighting gets you on paper without wasting ammunition. On an AR-pattern rifle, straight-pull or not, you can remove the bolt carrier, rest the rifle so it looks through the bore at a target 25 m away, and adjust the dot onto the same point. A laser bore-sighter does the same job on rifles where you cannot see through the bore.
Step 5: zero with live fire
- Set a target at a measured 25 m and fire a three- to five-shot group from a steady rest, aiming at the same point each time.
- Measure how far the centre of the group is from the aim point, and convert that to clicks using your optic's click value.
- Most turrets are marked with the direction the point of impact moves. Adjust, then fire a fresh group.
- When the group is centred at 25 m, move to 50 m and refine.
| Click value | At 25 m (27 yd) | At 50 m (55 yd) | At 100 m (109 yd) | Typical optics |
|---|---|---|---|---|
| 0.5 MOA | 3.6 mm (0.14 in) | 7.3 mm (0.29 in) | 14.5 mm (0.57 in) | SIG ROMEO5, Holosun HS510C, AEMS CORE and ARO EVO |
| 1 MOA | 7.3 mm (0.29 in) | 14.5 mm (0.57 in) | 29.1 mm (1.15 in) | Holosun HS507C X2 and many pistol dots |
So on a SIG ROMEO5 Gen II, which SIG specifies at 0.5 MOA per click, a group 29 mm low at 25 m needs about eight clicks up. For the arithmetic in more detail, see what is MOA?.
Which distance to finish at
For a .223/5.56 rifle, a 50 m zero keeps the bullet within about 5 cm (2 in) of the dot to roughly 200 m. A 25 m zero is quicker to set but shoots 15–21 cm (6–8 in) high between 100 m and 200 m. A .22 LR with the dot at AR height is effectively zeroed at both 25 m and 50 m at once. The full figures are in our red dot zero distance chart. Pistol optics are commonly zeroed at 10–25 m.
Expert tip
Only have 25 m available but want a 50 m zero on a .223? Zero the group about 3 cm (1.2 in) below the aim point at 25 m, then confirm at 50 m when you get the chance. The offset depends on your load and mount height, so treat it as a starting point.
Step 6: confirm and record
Fire a final confirmation group, then write down the distance, whether it was metres or yards, the ammunition and the mount height. Re-check screw torque after the first range session, because recoil can reveal a screw that was not fully seated. Witness marks in paint pen across screw heads make any movement obvious at a glance.
UK note
Short firearms (a barrel under 30 cm or overall length under 60 cm) and self-loading centrefire rifles are prohibited in England, Wales and Scotland under section 5 of the Firearms Act 1968. So British pistol-optic buyers are mainly airsoft players, gallery-rifle and long-barrelled pistol shooters, holders of Section 5 authority, and shooters in Northern Ireland, whose prohibited-weapons list has no equivalent of the short-firearm ban. Rifle dots here usually go on straight-pull, rimfire or airsoft AR-pattern rifles. For what applies to you, confirm with your police firearms licensing department.
Keeping your zero
- Correct torque and clean, locked threads prevent most lost zeros.
- Always push the mount forward in its rail slot before tightening.
- Re-zero if you change ammunition, mount height or the rifle it sits on.
- Look after the lens and battery contacts; our optic maintenance guide covers what to check.
Frequently asked questions
What distance should I zero a rifle red dot at in the UK?
Start at 25 m to get on paper, then finish at 50 m, which suits most .223 and 5.56 shooting on British ranges. If you only shoot a .22 LR at 25 m, a 25 m zero will also be close at 50 m.
Do I need thread locker on red dot screws?
Medium-strength (blue) thread locker plus correct torque is the usual approach, but follow the optic maker's instructions for your model. Never use high-strength (red) thread locker on optic screws you may need to remove.
How much does each click move the dot?
It depends on the optic. Many rifle dots use 0.5 MOA clicks (about 7 mm at 50 m), and many pistol dots use 1 MOA (about 29 mm at 100 m). The value is in the manual and usually printed on the turret.
Why does my red dot keep losing zero?
Usually loose or under-torqued screws, a mount not pushed forward in its slot, or oily threads that stopped the thread locker working. Strip it, degrease, re-torque and re-zero.
Shop this guide
- Rifle Dot Sights — many include a co-witness mount
- Pistol Dot Sights — open and enclosed optics for optics-ready slides
- Red Dot Sight Mounts — Trijicon RMR mounts and slide kits
