Aridus Industries CROM Optic Mount (Remington 870 / RMR)

The Aridus Industries CROM (Co-witness Ready Optic Mount) is the lowest, most rigid way to put a red dot on a Remington 870. Instead of a Picatinny rail mount that perches the optic well above the bore, the CROM bolts directly to the factory drilled and tapped holes on the top of the 870 receiver. This drops optic height by roughly 0.4 inches versus a typical Picatinny mount, putting the dot in line with the natural cheek weld instead of forcing a chin weld every time the gun comes up.

Compatibility note: the 870 RMR CROM only fits the four-hole Remington receiver pattern. After the Freedom Group bankruptcy reorganization, RemArms transitioned newer 870 production to a three-hole pattern that the original CROM does not support. Most pre-2020 870 Express, Police, Wingmaster, and Tactical guns are four-hole; verify yours by removing the factory rail or filler screws and counting tapped holes before ordering. The CROM also fits the 1100 and 11-87 with the same four-hole pattern, plus the VersaMax.

The mount accepts any Trijicon RMR footprint optic, which includes the Holosun 507C, 407C, 508T, and Holosun 507COMP as direct-fit alternatives. An integrated XS rear ghost ring sight gives an emergency backup sighting system: pair the CROM with an XS Big Dot tritium front sight (~$74), and the shooter has both an electronic dot and an iron co-witness on a single unit. If the optic battery dies or the dot fails, the ghost ring is immediately available without removing the optic. Aridus offers an optional Rob Haught modified rear ghost ring for $15 extra, which widens the aperture for faster acquisition under stress.

Installation uses the supplied screws and Loctite (do NOT skip the Loctite, the 870 receiver is heat-treated steel and threads strip slowly under recoil if the screws back out). The trade-off vs a Picatinny rail mount is versatility: the CROM is RMR-footprint only, so swapping to an Aimpoint Micro or Acro means buying a different mount. Aridus also makes an Aimpoint Micro CROM for shooters committed to that footprint, sold separately at the same price tier.