Pick your optic and mount to see the dot's exact optical axis height above the rail, whether it lands on a co-witness standard (1.41 absolute, lower 1/3, 1.93, or 2.26), and which riser fixes a mismatch with your magnifier. Every height comes from a manufacturer spec, manual, or verified listing; nothing is guessed. Picking the optic itself? Start with the best AR-15 red dots guide.
Enclosed 1x40 with one fixed base height: 1.41" absolute co-witness per the operator's manual. Sig offers no taller base, so reaching a 2.26" magnifier line requires a 0.85" rail riser under the whole optic.
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Model: AR-15 upper · Original AEMS reference body · height-adapted mount
Drag to rotate. Solid line: your dot. Dashed line: target height. Shapes adapt to the selected heights; they illustrate the stack rather than every product’s body or mount. All heights start at the rail top.
Your dot sits -0.85" relative to the target height.
Run absolute co-witness (1.41 inch) or lower 1/3 (1.57 to 1.66 inch) if you shoot a traditional cheek weld and want backup irons visible through the optic. Run 1.93 inch for a more heads-up stance, and 2.26 inch if you shoot over night vision, a gas mask, or a plate carrier. Many Unity FAST red-dot mounts use 2.26, while Scalarworks LEAP/01 covers 1.42, 1.57, and 1.93 on the Aimpoint Micro footprint.
Yes, by about 0.1 inch. The ROMEO4T shares the Aimpoint Micro footprint, so it drops into the same mounts as a T-2, but its body carries the optical center roughly 0.1 inch higher; side-by-side measurements put a 4T near 2.36 inches in a 2.26 inch Unity FAST Micro. Mount makers quote one centerline per footprint against the T-2 reference body, so taller-decked optics like the 4T ride slightly above the published number. That 0.1 inch will not break a magnifier pairing, the dot just sits a touch high in the magnifier's view.
Add a 0.85 inch Picatinny riser under the optic. Sig's operator manual puts the ROMEO8T-AMR's integrated base at a 1.41 inch absolute co-witness sight height, and Unity FAST FTC magnifier mounts center at 2.26 inches. Sig sells no taller base, so the fix is the Unity FAST Absolute Riser, which adds exactly 0.85 inch and lands the dot level with the magnifier.
Lower 1/3 co-witness places iron sights in the bottom third of the red dot window, keeping the upper window clear while preserving a backup sight picture. In practice it is a band, not one number: Scalarworks builds its lower 1/3 LEAP/01 at 1.57 inches, Holosun's 510C spacer lands at 1.63 inches, and Trijicon's MRO lower 1/3 mounts run 1.60 to 1.66 inches. Anything in that range reads as lower 1/3 on an AR-15.
A 1.93 inch mount raises the dot for a heads-up, squared stance and easier passive aiming through night vision, without going all the way to 2.26. Standard 1.41 inch iron sights no longer co-witness at 1.93; if you want backup irons you pair it with 1.93-height irons like the Scalarworks PEAK/02. The Reptilia DOT and Scalarworks LEAP/01 both offer dedicated 1.93 versions.
FAST Micro, AEMS, MRO, Comp Series, and MicroPrism mounts place their specified optics at a 2.26-inch centerline above the rail. FAST MRDS reaches that height with a matching optic adapter plate. Heights depend on the optic and mount: a rail riser adds to the existing sight height, and other FAST products use different heights.
The centerlines should match; that is the whole point of matched systems like a 2.26 inch dot with a 2.26 inch Unity FAST FTC magnifier mount, or Aimpoint's convention of pairing a 39mm sight height with a 39mm FlipMount magnifier. A small mismatch of a tenth of an inch or so leaves the dot off-center but still usable in the magnifier's view. Magnifiers do not have large vertical adjustment ranges, so fix real mismatches with mount or riser selection, not the magnifier.
Standard 1.41 inch AR-15 iron sights cannot; they sit far below a 2.26 inch window. That height is a deliberate heads-up choice, not a co-witness setup. The Unity FAST Micro includes adjustable standard-height backup sights in the mount body below the optic. Those sights provide a separate sight line through the mount.