An optic that will not bolt to the host is the most expensive kind of wrong. It is not faulty, it is not a return the manufacturer will entertain, and no amount of thread-locker will rescue it: the screw holes are simply in the wrong places. That pattern of holes, the locating posts around them and the flat deck they sit on is the optic's footprint, and it is the one specification that has to match before any other specification is worth reading.
It is also the specification buyers skip, because nobody markets it. Window size, dot size and battery life get top billing; the footprint is one line near the bottom of the spec table, labelled Footprint. Read it first.
What a footprint is, physically
Three separate things have to agree before an optic and a host are compatible, and a footprint is the name for all three at once.
First, screw spacing and thread: two fasteners, a set distance apart, in a set size. Second, the locating features — posts, lugs or a recess machined into the deck, sitting in matching holes in the cut. These do the real work. Screws clamp the optic down; the posts stop it walking backwards under recoil, and an optic held by screws alone will shift zero and eventually shear them. Third, deck geometry: the shape and height of the flat the optic sits on, which decides how high the glass ends up.
Two optics can be near-identical in size and share none of that. Window dimensions tell you nothing about fitment.
The patterns you will meet
Trijicon RMR is the full-size pattern with the broadest support. It is what the RMR Type 2 uses, and Trijicon has kept it on the newer designs: the enclosed RCR mounts through what its spec table calls a patented mounting system based on the RMR footprint, and the RMR HD keeps it outright despite the larger window and top-loading battery. Holosun builds a long list of optics to the same pattern, and SRO-footprint mounts are cut to take RMR optics too.
Shield RMSc and Holosun K are the compact pair, and they are related rather than identical. Holosun's own spec tables describe the K footprint as a modified Shield RMSc — that word "modified" is the whole story, and it is why K-footprint optics so often arrive with an adapter plate in the box. SIG's ROMEO-X Compact, by contrast, states plain RMSc. The store groups both under one RMSc / Holosun K footprint heading because the hosts overlap heavily, but the two lines on the spec sheets are not the same line.
Leupold DeltaPoint Pro is the other widely milled full-size pattern. The ROMEO-X Pro is cut for it, and so is the Holosun SCS 320, whose data sheet says it drops onto SIG P320 and DeltaPoint Pro-footprint slides with no plate between optic and steel. Both sit in the DeltaPoint Pro footprint group.
Aimpoint's ACRO is worth knowing by name, but this range is built around Trijicon, EOTech, Holosun and SIG Sauer, so an ACRO-cut host needs an optic from a dealer carrying Aimpoint.
Which optics fit which cut
| Footprint pattern | Optics in this range that use it | What the product data says |
|---|---|---|
| Trijicon RMR | Trijicon RMR Type 2, RMR HD, RCR, SRO; Holosun 407C and 507C (X2 and X3), 508T, 407COMP and 507COMP, 507 ProMax, Ronin 507COMP | “Footprint: Trijicon RMR” |
| Holosun K (modified Shield RMSc) | Holosun 407K X2 and 507K X2, EPS, EPS Carry, Ronin EPS Carry, SCS Carry | “Footprint: Holosun K (modified Shield RMSc)” |
| Shield RMSc | SIG Sauer ROMEO-X Compact | “RMSc (P365 / Shield RMSc)” |
| Leupold DeltaPoint Pro | SIG Sauer ROMEO-X Pro; Holosun SCS 320 | “Footprint: DeltaPoint Pro” |
| Holosun 509T multi-plate | Holosun 509T X2 in red, green and gold | “Holosun 509T (multi-plate system; includes RMR adapter plate)” |
| Direct mount, cut for one host | SCS MOS (Glock MOS), SCS 320 (SIG P320), SCS VP9 (H&K VP9), SCS MP2 (S&W M&P M2.0), SCS P10 (CZ P-10), SCS PDP (Walther PDP 2.0) | “Direct mount for … (no adapter plate)” |
| Picatinny rail — no pistol footprint | Holosun AEMS EVO, Holosun ARO and rail-mounted rifle optics generally | A mount is supplied; there is no footprint line |
The K-footprint trap
This is the detail that catches people out. The full-size Holosun EPS and the shorter EPS Carry are both Holosun K footprint. They are not a full-size pattern and a compact pattern. They differ in body length and window size, not in mounting standard.
What differs is the plate in the box. The EPS ships with an RMR-to-K adapter plate, so a K-footprint optic can go onto an RMR cut. The EPS Carry ships with an RMSc-to-K adapter plate, so the same footprint can go onto an RMSc cut. Order the wrong one on the assumption that "EPS" and "EPS Carry" are two different footprints and you will end up with the right pattern and the wrong bridge. Read the What's Included list, not just the footprint line.
The 507K X2, the single-dot 407K X2, the Ronin EPS Carry and the solar SCS Carry all sit on the same K pattern.
The 509T is its own thing
The Holosun 509T X2 is its own pattern. The footprint line reads "Holosun 509T (multi-plate system; includes RMR adapter plate)": it reaches an RMR cut through that plate rather than by bolting straight down. For a titanium Holosun that sits natively on an RMR cut, the open-emitter 508T X2 is the one.
Where the spec sheet stops short
Not every product page names a pattern. The SIG ROMEO-RS Compact says only that it "fits common compact optic cuts", with no footprint line at all. That is not an oversight to guess your way around. Where the manufacturer-verified data does not state a footprint, this article will not invent one — check with the pistol's manufacturer and with SIG before ordering.
What an adapter plate really costs
A plate solves a mismatch, and it charges you for it three ways.
Height. Every plate raises the glass, which can turn an absolute co-witness into a lower-third or into none at all. Work out the stack before you buy — co-witnessing a red dot with iron sights covers the heights.
Another joint. Two screwed interfaces instead of one, and the plate-to-host screws are the ones you cannot inspect without pulling the optic. A shifting zero on a plate-mounted optic is usually the plate, not the sight.
Screw length. This one is quietly responsible for a lot of damage. An RMR ships with the long screws intended for rail mounting; a milled cut wants shorter screws and a sealing plate underneath. Trijicon sells exactly that combination as the RMR footprint mounting kit for Glock MOS, Springfield OSP and Prodigy, Walther PDP and H&K VP9 cuts. Long screws in a shallow cut bottom out and crack housings.
The way to avoid all three is a direct-mount optic. Each Holosun SCS is machined for one host with nothing underneath it: SCS MOS for Glock MOS, SCS MP2 for the S&W M&P M2.0, SCS P10 for the CZ P-10, SCS VP9 for the H&K VP9, SCS PDP for the Walther PDP 2.0. The trade is flexibility: sell the host and the optic does not follow.
Identifying the cut in front of you
Four checks, in order of reliability.
- The cover plate. Most optics-ready pistols ship with a blanking plate over the cut, frequently stamped with a number or pattern name. That marking is the most direct answer you will get.
- The manufacturer's plate chart. Pistols supplied with a set of adapter plates come with a chart mapping each plate to the optics it accepts. Work backwards from it.
- Count and measure. Two screws with locating posts outboard of them on a wide deck points to a full-size pattern; a narrow deck with posts close in, to a compact one. A shortlist, not a confirmation.
- Ask the manufacturer. A twenty-minute email beats a returned optic.
Airsoft replicas deserve a note of their own, because they very often carry RMR-pattern cuts on the slide or a moulded mount. That opens the whole RMR footprint range to a replica, from the 507C X2 to the competition-window Ronin 507COMP. Replica machining varies, so check the fit dry before torquing anything down.
The hosts themselves
Pistol shooting in Great Britain does not look like it does in American writing on this subject, and the hosts are different. Long-barrelled pistols and revolvers may be held on a firearm certificate where their dimensions exceed those stipulated in section 5(1)(aba) of the Firearms Act 1968. Alongside them sit air pistols and airsoft replicas, and Northern Ireland has its own firearms legislation (Guide on firearms licensing law, s.4.5).
An airsoft gun is not a firearm. Section 57A of the Firearms Act 1968, inserted by the Policing and Crime Act 2017, defines it as "a barrelled weapon of any description which (a) is designed to discharge only a small plastic missile" and not capable of exceeding the permitted kinetic energy at the muzzle — 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. The missile must be made wholly or partly from plastics, be spherical, and not exceed 8 millimetres in diameter.
For air weapons, the Guide on firearms licensing law sets the thresholds at 12 ft lb for an air rifle and 6 ft lb for an air pistol (s.2.46). In Scotland, section 2 of the Air Weapons and Licensing (Scotland) Act 2015 makes it an offence to use, possess, purchase or acquire an air weapon without an air weapon certificate issued by Police Scotland (s.2.52). Northern Ireland has its own firearms legislation (s.4.5). None of that turns on the optic, but confirm anything uncertain with your police firearms licensing department or the relevant authority.
Rifle optics do not have a pistol footprint
A rifle-platform optic such as the AEMS EVO or the compact ARO has no pistol footprint at all: the AEMS EVO ships with a four-post lower-third co-witness mount, the ARO with a Picatinny mount. There is no footprint line on those pages because there is nothing to state.
The traffic runs the other way. An RMR-footprint optic can be moved onto a rail with the low RM33 or the taller RM34, both cut for RMR and SRO. Trijicon ACOG mounts for RMR optics are carried too; the ACOGs themselves are not part of this range.
Before you order
Confirm the host's cut from the cover plate or the manufacturer, then confirm the optic's Footprint line matches. If it does not, decide whether a plate is acceptable given the height it adds, and check it is supplied rather than assumed. Only then worry about dot size — what MOA means is the next question, zeroing the one after.