How do I tell if a red dot will fit my pistol?
Match the optic's footprint to your slide cut. Pick your pistol on the left and read the verdict on each optic: Direct mount bolts straight to the slide cut, Plate required means you also need an adapter plate, and Plate options means a plate is recommended to align footprints. The cards list the exact plates that bridge each pistol-and-optic pair.
Are RMR and RMSc the same footprint?
No. The Trijicon RMR (full-size) and the Shield RMSc (micro) are two different patterns with different hole spacing and recoil-lug shapes. RMR-cut slides (most full-size optic-ready pistols, plus Glock MOS with the RMR plate) will not accept an RMSc optic without an adapter, and an RMSc-cut subcompact like the P365 or Hellcat will not accept an RMR-footprint optic at all without slide milling.
Is the Holosun K footprint the same as RMSc?
Close, but not identical. The screws and overall dimensions match, but the lug count is different. The lugs sit on the slide and the pockets are recessed into the bottom of the optic. A true RMSc slide cut has 4 lugs and an RMSc optic has 4 matching pockets. The Holosun K footprint (407K, 507K, EPS Carry) only has 2 pockets. So when you put a 2-pocket Holosun K optic onto a 4-lug RMSc-cut slide like the Sig P365 or Hellcat OSP, 2 of the slide's lugs have nowhere to seat and hold the optic up. The standard fix is to file those 2 extra lugs off the slide so the K optic seats flush, or use the adapter plate Holosun ships with most K-series optics, which mounts to the slide and presents a clean K-pattern surface to the optic without touching the gun. The reverse direction (a 4-pocket RMSc optic on a 2-lug K cut) drops in without modification because the 2 extra pockets are simply unused.
What footprint fits a Sig P365?
The Sig P365, P365 XL, P365 X-Macro, and Hellcat OSP all use the Shield RMSc (micro) footprint. Sig markets their version as the ROMEOZero footprint, but it is dimensionally compatible with RMSc optics like the Holosun 507K / 407K / EPS Carry, Sig Romeo Zero / RomeoZero Elite / Romeo-X Compact, Swampfox Sentinel, and Trijicon RMRcc with the right plate.
What footprint fits a Glock MOS?
Glock MOS uses Glock's proprietary plate system, not a single footprint. Modern G19 / G17 / G45 / G47 MOS slides ship with a plate set that supports the Trijicon RMR, Leupold DeltaPoint Pro, Docter / Noblex, and C-More STS2 footprints, plus Aimpoint Acro on newer variants. Pick the plate that matches your optic's footprint and bolt it to the slide first, then mount the optic to the plate.
Can you mount an RMR on a Glock MOS?
Yes. Use the Trijicon RMR / Holosun 507C plate from the factory Glock MOS plate set, or an aftermarket steel plate from C&H Precision, Forward Controls, or ZEV. Any optic that shares the RMR footprint (Holosun 507C / 508T / 407C, SRO, Swampfox Justice, Riton 3 TAC RMD, Sig Romeo1Pro) will bolt to that same plate.
What optic mounts directly to a Glock 19 MOS?
Nothing mounts directly to a stock Glock MOS slide. Every optic on a factory MOS pistol sits on top of an adapter plate from the included plate set, even if the optic shares the RMR footprint. Aftermarket conversion plates from C&H, Forward Controls, or Holosun (the 509T-style MOS plate) let you dedicate the slide to one optic and get the lowest possible mounting height.
What is the difference between Holosun K and Holosun C optics?
The letter is the footprint. Holosun K-series optics (407K, 507K, EPS Carry) use the Shield RMSc micro footprint and fit subcompacts like the P365, Hellcat, and Glock 43X / 48 MOS. Holosun C-series optics (407C, 507C, 508T) use the full-size Trijicon RMR footprint and fit compact and full-size pistols like the Glock 19 MOS, P320, M&P 2.0 OR, and CZ P-10 OR via the RMR plate.
What does an adapter plate do?
An adapter plate sits between the slide cut and the optic so two different footprints can mate securely. It also fixes the optic's recoil lug to the slide and sets the mounting height. Each match in this tool lists the specific plates that bridge that pistol and that optic; install the plate to the slide with thread locker, then mount the optic to the plate.
Can I mount a red dot on a pistol that isn't optic-ready?
Only after slide milling. A non-optic-ready slide has no cut, no mounting holes, and no recoil shelf, so any direct-mount or plate solution requires sending the slide to a machinist for an RMR, RMSc, or Acro cut, or buying an aftermarket optic-cut slide. Toggle off Optic-ready only in this tool to browse non-cut models, then plan the mill or move to an MOS / OR / OSP / RDP / CORE factory variant.
Are MOS, OR, OSP, RDP, and CORE the same thing?
They all mean optic-ready, but the cut differs by brand. MOS is Glock's plate-system slide. OR is Smith & Wesson and CZ's name for a factory optic cut. OSP is Springfield's branding (Hellcat OSP, XD-M Elite OSP). RDP is Sig and HK's red-dot-pistol designation. CORE is Smith & Wesson's older M&P optic-ready system. All accept red dots, but each uses different footprints and plates, which is exactly what this checker resolves.
Do iron sights still work with a pistol red dot installed?
Only if you install suppressor-height iron sights. Standard factory sights sit too low to be seen through the optic window once a red dot is mounted. Suppressor-height (or optic-height) front and rear sights raise the sight plane so the irons co-witness through the red dot, giving you a backup if the dot fails or the battery dies. Most builds need both new front and rear sights, not just the front.
Why does my pistol show no compatible optics?
Either the pistol is not optic-ready, or the catalog has no plate path for its slide cut. Most factory non-optic-ready pistols need slide milling before any red dot will mount. Toggle off Optic-ready only to confirm whether your pistol has a factory cut, and if it does but still shows no matches, the slide footprint is uncommon enough that a custom plate or mill is the realistic path.