2 setups · 3 standard heights · which works with a magnifier
The AR-15 community has standardised on three mount heights: 36mm absolute, 38mm between, and 40mm lower 1/3.
What does co-witness actually mean?
When you look through a red dot sight, you see two things: the LED dot (a virtual image at optical infinity) and the world through the lens. If your mount is at the right height, you also see the iron sights' front post and rear aperture in the same view. That shared view is the co-witness.
The two co-witnesses put the iron post in different parts of the red dot's window. The practical difference: in absolute co-witness, the iron post sits dead-centre, lined up with the dot; in lower 1/3, the iron post is in the lower third of the window, and the dot sits in clean glass with iron sights visible but not occluding the target. The choice is height — 36mm vs 40mm in standard mounts.
Which one should you pick?
It depends on whether you run a magnifier and what you want to see when the red dot dies.
| Feature | Absolute co-witness | Lower 1/3 co-witness |
|---|---|---|
| Optic centerline height | ~1.4-1.42 in / 36mm above the rail | ~1.5-1.6 in / 38-40mm above the rail |
| What you see through the optic | Front post dead-centre, lined up with the dot | Front post in the lower third, dot in clean glass |
| With a magnifier flipped in | Iron post is centred in the magnified view — can occlude the target | Iron post at the bottom of the magnified view — target stays clear |
| Cheek weld on a standard AR-15 stock | Lower mount = more natural cheek contact | Higher mount = head lifts slightly, still ergonomic with proper stock |
| Best for | Traditional red-dot-only setup, no magnifier | Modern red dot + magnifier, holographic, or LPVO in upper 2/3 |
Quick rule: if you run a magnifier, choose lower 1/3 (40mm). If you run red dot only, absolute (36mm) keeps the cheek weld natural. The modern industry default has shifted to lower 1/3 since ~2015 because most AR-15s now run a magnifier or LPVO.
The 3 standard heights (36/38/40mm)
Three heights map to three use cases. The middle one (38mm) is a hybrid — close to absolute, but with the iron post already drifting toward the lower third. Pick by your optic, not by what your friend runs.
| Height | Co-witness type | Best for |
|---|---|---|
| 36mm (1.4 in) | Absolute | Red dot only, no magnifier. Cheek weld on a standard A2 or collapsible stock. |
| 38mm (1.5 in) | Hybrid / "1/2 co-witness" | Red dot with light magnifier use, or holographic sights that need the upper portion of the window clear. |
| 40mm (1.6 in) | Lower 1/3 | Red dot + 3x magnifier combo, EOTech-style holographic, or LPVO at 1x. |
There's also a fourth height: 1.93 in / 49mm ("heads-up"), used when you need clearance for a large objective bell (LPVO at high magnification) or a night-vision mount in front of the optic. That's outside the red-dot-magnifier co-witness story; it's a different mount family.
Where CVLIFE magnifiers fit in each height
The CVLIFE red dot and magnifier collection ships magnifiers in all three standard heights. Three representative options, all flip-to-side:
- For 36mm absolute co-witness: the CVLIFE JackalHowl 3X Red Dot Magnifier on a 36mm mount base. 2.95" eye relief, IP67, 1000G shockproof. Pairs with a low-profile red dot for an absolute co-witness setup without a magnifier occluding the target.
- For 38mm hybrid: the CVLIFE Multi-Reticle Red/Green Dot & 3X Magnifier Combo with 0.39-0.79" spacing between the red dot and magnifier. 2.95" eye relief. The combo's spacing is the variable that lets you tune between absolute and lower 1/3 without changing the mount.
- For 40mm lower 1/3: the CVLIFE Eaglefeather 3X/4X/5X Compact Magnifier with Mount on a 40mm mount. 3.54" eye relief. It effectively alleviates eye fatigue and provides a more comfortable viewing experience.
