Buying Guide

AR-15 Optic Mount Height: 1.54in, 1.93in & Cowitness Explained

6 September 2026 7 min read

Every AR-15 optic mount is sold with a number attached to it — 1.54in, 1.93in, 0.525in — and those numbers decide more about how your rifle feels than almost anything else you can bolt on. Get it right and the rifle comes up with the dot already in the window. Get it wrong and you spend every string of fire hunting for it.

This guide explains what those numbers measure, why two manufacturers can print different figures for what sounds like the same thing, and which height suits which job. Every mount referenced is one we stock.

What bore height actually measures

Bore height — also called optical axis height or height over bore — is the distance from the top of the Picatinny rail to the centre of the optic's window. It is not the height of the mount itself, and it is not measured from the barrel.

That single dimension sets three things at once:

Head position. A taller mount lets you keep your neck more upright behind the rifle. A lower mount pulls your cheek down onto the stock.
Whether your iron sights appear in the optic window. This is what "co-witness" means.
Offset at close range. The optic sits above the bore, so at very short distances the round strikes below your point of aim by roughly the bore height. A 1.93in mount puts the round about two inches low at the muzzle; a 1.54in mount about an inch and a half.

Absolute co-witness, lower 1/3, and why the numbers disagree

Absolute co-witness means your back-up iron sights sit in the middle of the optic window, with the dot landing on the front post. Lower 1/3 means the irons sit in the bottom third of the window, leaving a clear view above them for the dot. Anything taller than that and the irons drop out of the window altogether.

Here is the part that causes most of the confusion: co-witness is a relationship between three things — the mount, the optic's own window height, and your iron sights — not a property of the mount alone. That is why you will find one manufacturer calling 1.54in "lower 1/3" and a general reference chart listing something close to 1.5in as absolute co-witness. Both can be right, because they are talking about different optics.

The practical answer is to read the mount maker's own figure for the optic you actually own, which is why our product pages state it explicitly rather than leaving you to a generic chart.

The heights we stock, and what each is for

1.54in / 39mm — lower 1/3 co-witness

Reptilia's 39mm DOT mounts hold the optic at what the trade lists as 1.54in bore height, and standard AR-15 back-up iron sights co-witness in the lower third of the window. This is the default choice for a general-purpose rifle: irons still usable if the optic dies, dot clear above them, cheek weld close to a traditional stock position.

1.63in / 41mm — lower-third, slightly taller

The BCM A/T Optic Mount for Aimpoint Micro T2 is offered at 1.63in (41mm) as well as 1.93in. Both versions take the same optic, so the choice is height alone — 1.63in gives lower-third co-witness with standard AR-15 irons.

1.93in / 49mm — head-up, above co-witness

At 1.93in the sightline is raised for a more upright head position: easier to hold, easier to hold consistently, and far easier to work under night vision or with eye protection and a helmet. Standard AR-15 back-up iron sights do not co-witness at this height — they sit below the window. That is a deliberate trade, not a fault, and it is the height that has become standard on modern service-pattern rifles.

Lower than 1.54in

For a rifle with no back-up irons, or a host where the optic needs to sit as low as it will go, there is the Reptilia DOT Low Height Optic Mount for Aimpoint ACRO & Steiner MPS. The Daniel Defense Micro Mount takes a different approach again and ships with a lower 1/3 spacer in the box, so you pick absolute or lower 1/3 at the bench rather than at the checkout.

One footprint, one height, no adjustment — and why that is the point

Most of the mounts above are one-piece units cut for a single optic footprint at a single height, with nothing on them that adjusts. That is not a limitation, it is the design goal: a mount with no adjustable joint has nothing in it that can work loose, which is exactly what you want holding a zero.

The corollary is that footprint matters as much as height. An Aimpoint Micro will not fit an ACRO-footprint mount, a Trijicon MRO will not fit an RMR mount, and the Holosun 509T uses an entirely different enclosed footprint again. Check the footprint first, then choose the height.

Risers: changing the height you already have

A riser sits between the rail and your existing mount and adds its own thickness to whatever bore height you were running.

BCM A/T Optic Riser 0.525" — takes a lower-third optic to roughly 2.16in and an absolute-co-witness optic to roughly 1.93in.
Reptilia RMU Optic Riser 0.625" — for EOTech, Vortex UH-1 and Sig ROMEO8T sights.
Reptilia RMU Magnifier Riser 0.39" — raises a 3x or 6x magnifier to match a raised optic.
ROF Riser 2.5mm and ROF Riser 10mm for Trijicon RMR and SRO — fine adjustment on top of a ROF mount.

If you raise the primary optic, raise the magnifier with it. A magnifier at the wrong height simply does not line up, and it is the single most common mistake in a magnified red dot setup.

Scope mounts: ring size and bore height are two separate decisions

On a magnified optic you have two numbers to satisfy, and they are independent of each other. Ring size follows your scope tube — 30mm, 34mm or 35mm. Bore height follows your cheek weld — how high your comb sits and where your head naturally lands.

The Reptilia AUS range covers the grid directly, in 1.54in, 1.70in and 1.93in bore heights:

30mm: 1.54", 1.70", 1.93"
34mm: 1.54", 1.70", 1.93"
35mm: 1.54"

For a bolt-action rifle the Reptilia AGS Scope Mount 1.3" 20 MOA sits lower and adds 20 MOA of built-in elevation, which buys back adjustment range in the scope for long-range dialling. There is also a bubble level for the AUS mount, which matters more than most people expect once you are dialling elevation.

Browse the full range in precision rifle scope mounts.

Offset mounts: a second sightline without stripping the rifle

A 45-degree offset mount gives you a close-range aiming option without taking the magnified optic off. Roll the rifle 45 degrees and the dot is already there. Reptilia's ROF-45 mounts do this from a 30mm or 34mm scope ring, and the DOT and AUS offset mounts do it from the mount body itself.

See all of them in offset optic mounts & sights.

How to choose, in order

Footprint first. Aimpoint Micro, ACRO, RMR/SRO, MRO, 509T and AEMS are all different. Nothing else matters if the optic does not bolt on.
Do you want the irons in the window? Yes, and clear of the dot: 1.54in lower 1/3. Yes, centred: absolute co-witness. No: 1.93in.
Are you shooting with a helmet, ear defenders that push your head up, or night vision? Go 1.93in.
Magnified optic? Ring size from the scope tube, bore height from your cheek weld — and check the comb height your stock can actually reach.
Adding a magnifier later? Buy the matching riser at the same time.