Glock's MOS (Modular Optic System) slides are milled for optics but almost never fit one directly — you need the right adapter plate for your optic's footprint. Get the plate wrong and the optic either won't bolt down or will shoot loose. Here's the match-up.
Step 1 — identify your optic's footprint
The footprint is the pattern of screw holes and recoil bosses on the optic's base. The common ones: RMR (Trijicon RMR, Holosun 407C/507C/508T), K footprint (Holosun 407K/507K, EPS, EPS Carry — closely related to RMSc), and Holosun 509T (its own lug pattern). Full reference: our footprint compatibility chart and RMR vs RMSc explained.
Step 2 — match the plate
| Your optic | Footprint | Plate for Glock MOS 9mm |
|---|---|---|
| Holosun 407C / 507C / 508T, Trijicon RMR | RMR | Glock factory MOS plate 01 (in the box) |
| Holosun 407K / 507K / EPS / EPS Carry | K | Holosun KPLT-MOS9MM |
| Holosun HS509T / HE509T | 509T | Holosun 509PLT-MOS9MM |
| Shield RMS / SMS class | RMSc | Glock factory MOS plate 04 |
Genuine Holosun plates beat generic ones for a simple reason: correct recoil-boss engagement machined for the exact optic, with the right screws in the bag. Browse all mounting plates and Holosun mounts & plates.
This guide covers the fitting, not the licensing. Handguns are held very differently across the UK — in Great Britain a firearm certificate cannot be granted for self-defence, and the MOS pattern is most often met here on airsoft replicas, long-barrelled pistols and pistol-calibre carbines. Optics themselves need no licence anywhere in the UK: see do you need a licence for a rifle scope, red dot or torch.
Step 3 — fit it properly
- Degrease everything — screws, plate, slide threads — before assembly.
- Blue (medium) threadlocker on plate-to-slide and optic-to-plate screws. Never red.
- Torque: follow the optic maker's spec (typically around 15 in-lb for optic screws — check your manual; snug + threadlocker beats gorilla-tight).
- Screw length check: optic screws that are too long bottom out in the slide and leave the optic loose while feeling tight. If the optic rocks after torquing, stop and check.
- Witness-mark the screws with a paint pen, then re-check after the first 100 rounds.
Step 4 — zero it
Full walkthrough in how to mount and zero a red dot. Confirm zero at 10 and 25 metres.
Not sure which optic to start with? See best pistol red dots and our guide to compact pistol red dot sights, or browse all pistol dot sights. Duty paid, with £5 tracked UK delivery.
