Two optics in this shop ship on a mount of the same published height, and their specifications disagree about what that height gives you. The Holosun HS510C arrives on a quick-detach mount listed at 1.41 in (35.8 mm), described as lower-1/3 co-witness. The SIG Sauer ROMEO5 Gen II Elite includes a 1.41 in (35.8 mm) Picatinny mount, described as absolute co-witness with standard AR iron sights. Same figure, opposite labels.
Neither listing is wrong, and that contradiction is the most useful thing on this page. Co-witness is not a feature an optic has; it is what happens when one height meets another — the optic's line of sight against the iron sights of whatever it is bolted to. Treat it as a mounting-height problem rather than a sighting technique and the label on the box stops being the thing you buy on.
What co-witness actually describes
A red dot co-witnesses when the iron sights are visible through its window and line up with the dot, rather than disappearing under the housing or sitting above the glass. If the dot fails — flat battery, dead emitter, a lens full of mud — you keep shooting through the same window on the irons, with nothing stripped off the rail.
Two conditions have to hold. The iron sight line has to pass through the window, which is a question of how tall the irons are and how high the optic sits. And the two systems have to agree on where they point, which they do at whatever distance both are zeroed at. Inside and beyond that distance they drift apart slightly, because dot and irons sit at different heights above the bore. At the distances a 1x optic is used, that divergence is small — worth knowing about rather than worrying about.
Absolute and lower-1/3: where the irons land, and what each costs
Absolute co-witness puts the iron sight line and the optic's line of sight on the same level. Look through the glass and the front post sits centred, right where the dot is. If the dot vanishes, the picture you already have is the iron sight picture — no change of head position at all. The cost is that post and notch live in the middle of your view permanently, which behind a circle-dot reticle makes for a busy window.
Lower-1/3 co-witness raises the optic so the irons fall into the bottom third of the window. The middle of the glass is clear and most of the sight picture is target. You pay for it in head position: the sight line sits higher above the bore, so you hold your head more upright behind the stock. Many shooters prefer that, particularly where a helmet or a night-vision device makes a crushed cheek position awkward.
It is a genuine trade rather than a right answer. If the irons are the serious backup and you want no transition at all, absolute earns its clutter. If they are a formality that folds away and speed through clean glass is the point, lower-1/3 suits how most people actually shoot a 1x optic.
Why the same 1.41 in produces two different answers
Three separate vocabularies are in use across the optics in this catalogue, and they do not describe the same quantity.
Some publish a height figure — the HS510C and the ROMEO5 Gen II Elite both at 1.41 in (35.8 mm). Some publish added riser height: the EOTech EXPS3-0 lists an integrated quick-detach lever on a 7 mm (0.28 in) raised base at lower-1/3 co-witness — height added, not total. Some publish words only: the Trijicon RM34 is Picatinny at raised height, its sibling the RM33 simply the low one.
The reason for the disagreement is in the SIG wording itself: 1.41 in is absolute co-witness with standard AR iron sights. The label is a prediction about a host. Change the host — different irons, a receiver and gas-block rail that are not on one plane, an airsoft replica whose sights are not built to AR dimensions — and the same mount gives a different result. Makers also do not all state where the height is measured from and to. Two honest specifications, one number.
Take the figure as the fact and the label as an assumption about a rifle that may not be yours. What this shop's optics and mounts actually publish:
| Optic or mount | Supplied in the box | Height published |
|---|---|---|
| Holosun HS510C | Quick-detach co-witness mount | 1.41 in (35.8 mm), listed as lower-1/3 |
| SIG Sauer ROMEO5 Gen II Elite | Picatinny co-witness mount | 1.41 in (35.8 mm), listed as absolute |
| SIG Sauer ROMEO5 Gen II | Low mount and a co-witness riser | Not published |
| EOTech EXPS3-0 | Integrated QD lever base | 7 mm (0.28 in) raised base, listed as lower-1/3 |
| EOTech XPS3-0 | 1 in Weaver / MIL-STD-1913 cross-bolt | Not published — no raised base |
| Holosun AEMS EVO and AEMS CORE X2 | Four-post lower-1/3 co-witness mount | Not published |
| Holosun ARO | Picatinny mount and hardware | Not published |
| Trijicon RM34 (RMR / SRO footprint) | Mount only | Not published — specified as raised |
| Trijicon RM33 (RMR / SRO footprint) | Mount only | Not published — specified as low |
Where no figure is published, the words are shorthand for a host the manufacturer had in mind. The EXPS3-0 and the XPS3-0 make the point plainly: same reticle, same optical package, and the only thing that makes one a co-witness height is a 7 mm (0.28 in) block of aluminium under the housing.
Which UK platforms this question even applies to
Co-witnessing assumes iron sights and a rail that holds the optic in line with them. Both are less universal than the internet suggests.
Airsoft replicas are where the question comes up most often. AR-pattern replicas generally have a full-length top rail and folding backup sights, so the absolute-versus-lower-1/3 decision is live. Airsoft guns are defined separately in law rather than as firearms: section 57A of the Firearms Act 1968, inserted by the Policing and Crime Act 2017, defines an airsoft gun as a barrelled weapon designed to discharge only a small plastic missile that is not capable of exceeding the permitted kinetic energy at the muzzle — 1.3 joules for a weapon that fires two or more missiles successively without repeated trigger pressure, and 2.5 joules in any other case.
Certificate-held rifles with flat-top receivers and rail-mounted sights are the other clear case. Where a licensing question arises, confirm it with your police firearms licensing department; Northern Ireland has its own firearms legislation, so the position there is not read off the position in Great Britain (Guide on firearms licensing law, s.4.5).
Air rifles usually are not a co-witness proposition. Most sit on an 11 mm dovetail rather than Picatinny, and many have no irons fitted at all — in which case a riser buys nothing but a higher dot. The same guide sets the thresholds at which an air weapon stops being an exempt air weapon — 12 ft lb for an air rifle, 6 ft lb for an air pistol (s.2.46) — and records that section 2 of the Air Weapons and Licensing (Scotland) Act 2015 makes it an offence to use, possess, purchase or acquire an air weapon in Scotland without an air weapon certificate issued by Police Scotland (s.2.52).
A pistol optic on a milled slide is not a co-witness question at all. The slide cut fixes the deck height; there is nothing to choose. The Holosun SCS MOS bolts straight to a Glock MOS slide with no adapter plate, which is exactly why its height is not a variable. Where a pistol dot has to line up with irons, the fix is taller iron sights, not a taller mount. We are an optics specialist and sell no iron sights of any kind, so those come from a gunsmith or general parts retailer; an 11 mm dovetail adapter is likewise not something we stock.
Folding irons change the argument
Fixed irons force the trade above. Folding backup sights largely dissolve it: down, they sit below the sight line and are irrelevant; up, they are the backup. That is why absolute co-witness is easier to live with than it sounds — the clutter is optional, one thumb movement away.
Two things still have to be checked. First, front and rear sights must sit on rails lying in the same plane: a folding rear on the receiver and a folding front on a gas-block section only agree if those rails are level, and no mount rescues a mismatched handguard. Second, rail space — a quick-detach base such as the EXPS3-0's occupies slots, a folding rear needs its own, and a magnifier behind the optic must have somewhere clear to swing.
What to check before you order
- Is the rail continuous and level? A monolithic flat-top is the easy case; separate receiver and handguard rails are what catches people out.
- What sight-line height do your irons give? This is the number the mount has to match. Without it, no co-witness label on any box can be judged.
- Does the optic ship with a mount, and is a height published? Several arrive on a co-witness mount already, and need nothing else bought.
- Is the height adjustable or the spacer removable? The ROMEO5 Gen II includes a low mount and a co-witness riser — two heights in one box. The Holosun HM3X magnifier's flip-to-side mount is specified with adjustable co-witness height, which matters because a magnifier must match the optic in front of it, not the rail.
- Does the footprint match the mount? Height and footprint are separate questions; our red dot sight footprints guide covers which optic fits which cut.
- Plan to re-zero. Changing mount height moves the sight line relative to the bore, so the old zero does not carry over — the procedure and its failure modes are in how to zero a red dot sight.
The test that settles it
Fit the optic on the mount you have, put the rifle up, and look. Front post centred with the dot is absolute. Post in the bottom third with clear glass above it is lower-1/3. No post visible at all means the optic is too high for those irons or the irons too short for the optic — and you now know which way to move, which is more than the wording on any packaging tells you.
Everything after that is fitment and procedure rather than height. Co-witness mounts mostly arrive with rifle dot sights and holographic sights; the red dot sight mounts range covers RMR and SRO optics that come without one. If you are unsure which side of the platform line you are on, pistol dot sight vs rifle dot sight is the piece for that.