The optic that gets sent back is rarely the wrong optic. It is the right optic with the wrong four screw holes underneath. Window size, reticle, sealing and runtime are preferences you can live with either way once the sight is on the gun. Fitment is not a preference: it bolts down or it does not. So this works outward from there — what mates to what, what an adapter plate genuinely fixes, and only then the choices you are free to make.
What a pistol optic gets fitted to in Britain
The word "pistol" does not mean here what it means in most optics marketing written elsewhere. UK pistol shooting is long-barrelled revolvers and pistols held on a firearm certificate, air pistols and airsoft pistols, and Northern Ireland has its own firearms legislation (Guide on firearms licensing law, s.4.5). Those three are, in practice, the platforms UK shooters mount pistol-pattern optics to:
- Long-barrelled revolvers and pistols held on a firearm certificate. The legislation permits these where their dimensions exceed those stipulated in section 5(1)(aba) of the Firearms Act 1968. The qualifying dimensions are a matter for your police firearms licensing department to confirm, not a retailer.
- Air pistols. The Guide on firearms licensing law puts the air pistol threshold at 6 ft lb, against 12 ft lb for an air rifle (s.2.46). Above that figure an air weapon is not an exempt air weapon.
- Airsoft pistols. Section 57A of the Firearms Act 1968, inserted by the Policing and Crime Act 2017, defines an airsoft gun as a barrelled weapon designed to discharge only a small plastic missile — spherical, made wholly or partly from plastics, not exceeding 8 mm in diameter — and not capable of exceeding the permitted kinetic energy at the muzzle. Those levels are 1.3 joules for a weapon that can discharge two or more missiles successively without repeated pressure on the trigger, and 2.5 joules in any other case.
Two jurisdictional points sit on top. Section 2 of the Air Weapons and Licensing (Scotland) Act 2015 makes it an offence for a person to use, possess, purchase or acquire an air weapon in Scotland without holding an air weapon certificate issued by Police Scotland (Guide, s.2.52). Northern Ireland has its own firearms legislation (Guide, s.4.5), so nothing above reads across to it. Confirm your own position with your police firearms licensing department or the relevant authority before committing to a platform, never mind an optic.
If the gun is actually a rifle or carbine, the optics are a different shape on a different mounting problem, and the rifle and general-purpose guide is the one you want.
Footprints: the decision everything else waits on
A footprint is the pattern of screw holes and locating posts on the underside of an optic and the top of the cut it drops into. Three patterns cover almost every pistol-pattern sight in stock, plus a fourth that sidesteps the question.
| Footprint | Typically cut for | Stocked optics on this pattern | What to watch |
|---|---|---|---|
| Trijicon RMR | Full-size optics-ready cuts, and most rail mounts | RMR Type 2, SRO, RCR, HS507C-X2, HS407C-X2, HS507COMP | The widest choice by a distance. A milled cut usually needs shorter screws and a sealing plate than the rifle-length hardware in the box. |
| Shield RMSc / Holosun K | Slim and subcompact cuts | HS507K-X2, HS407K-X2, EPS Carry, ROMEO-X Compact | Holosun K is a modified RMSc pattern, not the same pattern. Related, not interchangeable — check which one a sight ships plates for. |
| DeltaPoint Pro | Full-size cuts milled to the DPP pattern | ROMEO-X Pro, SCS 320 | A smaller catalogue than RMR, so settle the footprint before you fall for a model. |
| Direct mount (no plate) | Named factory optics-ready slides | SCS MOS, SCS 320 and the rest of the SCS range | Model-specific by design. Nothing to adapt, and nothing to reuse if you change platform. |
Two details in that table cost people money.
The first is that Holosun K is documented as a modified Shield RMSc pattern. Close relatives, not clones — which is why Holosun ships plates. The EPS Carry is a Holosun K optic with an RMSc-to-K adapter plate in the box. The full-size EPS is also a Holosun K optic, but ships with an RMR-to-K plate, because the cuts it realistically goes onto are RMR cuts. Two sights, same footprint, opposite plates — the included-hardware line is worth more than the review.
The second is that the RMR footprint is not one thing in the box. Trijicon supplies its sights with long screws suited to a rail mount; a milled cut wants short screws and a sealing plate underneath, which is what the RMR mounting kit is for. An RMR-cut gun and an RMR-footprint optic can still leave you a hex key and two screws short of a fitted sight.
What an adapter plate does and does not solve
A plate translates one hole pattern into another. That is all it does, and three things it does not do are worth knowing first.
It does not remove height. Every plate adds a layer between optic and gun, raising the sight line and changing whatever relationship you had with your iron sights. A sight advertised as low deck height stops being one once it sits on a plate.
It does not add rigidity. A plate introduces a second bolted joint, and that is a second thing to check when a zero starts wandering.
And it does not create a cut that is not there. On air and airsoft pistols with no milled recess, the answer is usually a dovetail or rail adapter — and that is a real gap here: the store carries RMR-footprint mounts and the Trijicon sealing-plate kit, but no 11 mm dovetail adapters or generic pistol-plate sets. An air pistol with a dovetail groove will need that bracket sourced from an airgun specialist.
The direct-mount route
The SCS family takes the opposite approach: no plate, because each version is cut for one named slide — SCS MOS for a Glock MOS slide, SCS 320 for a P320 or another DeltaPoint Pro cut. That removes the plate, the height and the extra joint together. The trade is portability: an SCS stays with its platform.
Sealed or open, judged in British weather
An open reflex sight leaves the emitter exposed in a well beneath the lens. Anything that settles there — drizzle, grit off a range floor, a smear from a wet glove — dims or blanks the dot until you clean it. An enclosed sight puts glass on both sides so nothing reaches the emitter at all.
Indoors, or on dry club days where a lens wipe is a minor annoyance, an open sight is entirely dependable and gives you more choice at a given budget. Shooting outdoors year-round, the sealed options earn their extra height.
The sealed pistol-pattern optics here fall into three groups. The Holosun EPS and EPS Carry are 7075 aluminium, rated IPX8, on a Holosun K footprint. The Trijicon RCR encloses the RMR's recoil-diverting housing shape in forged 7075-T6 at 1.69 oz (48 g), on the RMR footprint. The SCS models are enclosed too, direct-mounted. Open reflex vs enclosed covers the trade-off in depth.
Note that "sealed" and "waterproof" are separate specifications. The open-emitter RMR Type 2 is rated submersible to 20 m (66 ft) and the SRO to 3 m (10 ft) — water does not get inside either. What an enclosed design stops is debris landing on the emitter, which is a different failure and the one you meet on a wet range day.
Dot size against the targets these disciplines shoot
MOA is angular, so a 6 MOA dot is three times the width of a 2 MOA dot at any distance you care to name, and covers roughly nine times the area. That ratio is the whole decision.
Precision work at a fixed, known distance against a small scoring ring wants the finer end. The HS407C-X2 gives a single 2 MOA dot with nothing to configure; the Trijicon SRO at 2.5 MOA does the same job in a 0.98 × 0.89 in (25 × 22.5 mm) round window that keeps the housing out of your view.
Speed shooting against a large plate or an airsoft target wants the opposite: the subcompact HS407K-X2 pairs a 6 MOA dot with a 0.58 × 0.77 in (15 × 20 mm) window, because a small window is quicker to work with when the dot in it is bold.
If your shooting spans both, a switchable reticle beats a compromise dot size. The HS507C-X2 and HS507K-X2 switch between a 2 MOA dot, a 32 MOA circle and both together; the HS507COMP offers selectable 8, 20 or 32 MOA circles around its 2 MOA dot in a 1.1 × 0.87 in (28 × 22 mm) window. Fixed circle-dots exist too — the ROMEO-X Pro and ROMEO-X Compact both run 2 MOA inside a 32 MOA circle. What MOA means has the arithmetic.
Battery access is really a zero question
The least glamorous spec on the list decides whether a dead cell costs you five minutes or a re-zero. If the battery sits under the optic, changing it means unbolting the sight, and re-mounting is never perfectly repeatable.
Side-loading is the common arrangement on Holosun and SIG pistol optics — a CR1632 on the HS407C-X2, HS507C-X2, HS407K-X2, HS507K-X2 and the ROMEO-X pair, a CR1620 on the EPS and EPS Carry — with the sight bolted down throughout. Top-loading covers the Trijicon RCR and SRO, both CR2032, changed in place. The RMR Type 2 is the one to plan around: its CR2032 runs up to around four years at setting 4, but lives underneath.
Then there is not having a battery at all. The SCS models run an internal rechargeable cell charged by a solar panel, with roughly a 20,000-hour reserve for the dark — which removes a whole category of maintenance from a gun that spends months between outings.
Before you order
Four things, in order:
- Identify the cut, not the gun. The manufacturer's plate chart or the markings on the supplied cover plate will name it. Footprints explained covers the patterns, and the ranges are grouped by RMR, RMSc / Holosun K and DeltaPoint Pro so you can shop from the answer.
- Read the included-hardware line. Which plate, which screw length, whether a sealing plate is in the box — where the EPS and EPS Carry differ, and where an RMR on a milled cut catches people out.
- Decide sealed or open against where you actually shoot, then let that narrow the model list rather than the other way round.
- Pick the dot last. It is the only item here you can change your mind about without buying different hardware — on a multi-reticle sight, with a button.
Fit it, torque it to the figure in the instructions, zero it properly, then re-check the screws after the first outing. A pistol-pattern optic on the right cut with the right hardware is a very boring piece of equipment, which is the point of it.