Holosun 407K, 507K and EPS Carry on a Glock 43X MOS: Do You Need an Adapter Plate? (2026)

Short answer for the Glock 43X MOS: it depends on which slide cut your pistol has. The Holosun 407K, 507K and EPS Carry all use the Holosun K footprint. On the original Slimline MOS cut — the one Glock shipped on the 43X MOS, 48 MOS and 43 MOS from launch through most of 2025 — that footprint needs an adapter plate, because the slide has four recoil posts and a K-footprint optic only has two shallow recesses to receive them. On the revised MOS-K cut that Glock began shipping in late 2025, the same optics drop straight on with no plate. Nothing about the optic changed. The slide did.

That single fact explains most of the arguing you will find in forum threads about Holosun on the Glock 43X MOS — two people can both be right about the same optic and the same pistol model, because they are holding two different slides. This guide shows you how to tell yours apart in about 30 seconds, then gives the model-by-model answer for each cut.

📖 Part of our Glock 43X MOS Complete Red Dot Guide — optics, adapter plates, installation and troubleshooting.

There Are Now Two Different Glock 43X MOS Optic Cuts

Until late 2025 there was only one story to tell: the slimline MOS slide was machined for the Shield RMSc footprint, and anything that was not RMSc needed a plate. Then Glock quietly revised the cut. Night Fision, which makes sights for these slides, documented the change: the update applies to the Glock 43, 43X and 48, and the new configuration is called MOS-K, “modified to accept either Holosun K or RMSc footprints without the need for adapter plates.”

Glock did not rename the pistol. A 43X MOS bought in 2024 and a 43X MOS bought today are both called “43X MOS” on the box, in the catalogue and on Glock’s own product page — which, on both cuts, carries the same one-line note: “The G43X with the Modular Optic System (MOS) does not come with adapter plates.” On the original cut that sentence means you will have to buy one; on MOS-K it means you will not need one. Same sentence, opposite advice. Retail listings are already appearing for the new variant — Davidson’s exclusive SKU PX435Z201FRMOSK is advertised as “Optic Rdy K Cut” and the listing states plainly that the pistol “does not come with adapter plates as it is a direct mount.”

How to Tell Which Cut You Have in 30 Seconds

Two checks, either one is sufficient:

1. Read the item number on the case. Night Fision’s guidance is to check “the item number sticker on the handle of your pistol case. The standard cut will have ‘MOS’ and the MOS-K cut will include ‘MOSK’ near the end.”
2. Count the posts in the optic cut. Take the slide off, remove the MOS cover plate, and look into the recess. Four posts — two at the front of the cut, two at the rear — is the original Slimline MOS cut. Two posts is the MOS-K cut.

The post count is the check that actually matters, because it is the geometry the optic has to sit down onto. If you have already sold the box, or you bought the pistol used, the post count is your answer.

What Actually Changed Mechanically

The posts inside the optic cut are recoil posts. They take the shear load of recoil so the two mounting screws are not doing that job alone. An RMSc-footprint optic has four matching recesses in its underside, so all four posts land in a pocket and the optic sits flat. A Holosun K optic has only two recesses, and they are shallower. Put a K optic on the original four-post cut and the pair of posts with nowhere to go holds the optic up off the slide — it will rock, and the screws will be pulling the optic down onto two hard points.

The clearest confirmation comes from the shops that mill this change for a living. Wager Machine Works describes its conversion in one sentence: “The cut involves removing the rear posts on the factory MOS cut and shortening the front posts to allow the 407k/507k to sit flush.” C&H Precision offers the equivalent service and states the result plainly — the upgrade “will allow for direct mounting of Holosun 407k/507k/EPS/EPS Carry or Trijicon RMRcc optics without the use of an adapter plate over the existing optics cut.”

That is the same outcome the MOS-K cut delivers from the factory: keep the front pair, lose the rear pair.

One Detail Published Sources Get Backwards

This is worth flagging because you will meet both versions online. Night Fision’s otherwise excellent write-up describes the MOS-K cut as having “two recoil bosses at the rear.” Three independent sources point the other way:

• Wager Machine Works, which performs the conversion: rear posts removed, front posts shortened.
• Primer Peak, from hands-on modification: “On the RMSc footprint, we have 4 equal-height recoil bosses… On the Holosun K, we only have two, and they’re at the front.”
• Our own measurement, published in the Glock 43X MOS footprint breakdown: the K optic’s front recesses are about 0.029″ deep against a post height of roughly 0.040″ — a 0.011″ interference even where a recess does exist.

Practically it does not change what you buy. It does change what you look for when you count posts: on a MOS-K slide the surviving pair sits at the front of the cut, nearest the muzzle.

Which Holosun Fits a Glock 43X MOS? Model by Model

The slimline slide is narrow, so the first filter is physical size: only Holosun’s K-size and EPS Carry housings sit inside the slide width. The second filter is footprint, and that is where the cut you have decides whether you need a plate.

Holosun 407K X2

The HS407K-X2 is the single-reticle K optic: one 6 MOA dot, side-loading battery tray, Shake Awake. Holosun lists it on the K footprint. On an original Slimline MOS cut it needs a plate. On a MOS-K cut it direct mounts. This is the model behind the largest single block of unanswered searches we see for this pistol — people type “glock 43x holosun 407k” and get pages that never say which slide they are describing.

Holosun 507K X2

The HS507K-X2 shares the 407K’s housing and footprint and adds the multi-reticle system — 2 MOA dot, 32 MOA circle, or both. Mounting behaviour is identical to the 407K: plate on the original cut, direct on MOS-K. If you are choosing between the two, choose on reticle preference, not on fitment.

Holosun EPS Carry

The EPS Carry is the enclosed-emitter option — the LED sits inside a sealed housing, so pocket lint and carbon cannot obstruct it. Holosun’s own listing describes it as K footprint with an RMSc-to-K plate, which is exactly the situation this article is about: it is not an RMSc optic, it is a K optic that Holosun ships an adaptation for. On a MOS-K slide you can skip that plate entirely.

Note the distinction between EPS Carry and the full-size EPS. Only the Carry version belongs on a 43X MOS.

507C, 407C, EPS Full-Size and 509T — Why They Do Not Fit

These are full-size housings built around the Trijicon RMR footprint or Holosun’s own larger patterns. They overhang the slimline slide on both sides, which creates snag points on the draw and rules out most holsters. No plate fixes a width problem. If you want an RMR-footprint optic on this pistol, the honest answer is a different pistol or a custom mill, not an adapter — we cover that trade-off in the RMR footprint guide.

Do You Actually Need an Adapter Plate? What 135 Returns Say

We sell an adapter plate for this pistol, so treat the next paragraph with that in mind — it argues against a sale more often than for one.

Across the full life of our AC311 adapter plate we have 135 recorded returns, 88 of them with a written customer comment. Two return reasons dominate the information-failure side of that pile: ORDERED_WRONG_ITEM (22) and NOT_AS_DESCRIBED (18). That is 40 of 135, or 29.6%, of all returns coming from people who did not know what they needed before the box arrived. It is not a manufacturing problem. It is a research problem, and this page is part of the reason it exists.

The most useful single comment in the set is from a customer who did the homework we should have done for them:

“The plate didn’t fit my specific model of Glock 43x, I reached out to the company and apparently Glock changed the slides which now require no adapter… it just won’t fit the newest model Glock 43X, would still work on an older model.”

Another put it in three words: “Says it fits 43x MOS, does not fit new optic mounts.” A cluster of others never got that far and simply reported the outcome — “Optics purchased ultimately did not require adapter plate”, “My slide didn’t need the spacer, package was never opened”, “I did not need adapter, optic fit without it.”

A separate, smaller cluster describes the opposite failure — a plate whose rear edge fouls the cut: “There was a lip at the back that did not allow it to fit”, “It has an extra protruding piece that won’t fit my slide.” Those are the reports we would expect from a plate designed around four posts meeting a slide that no longer has the rear pair.

So: check your cut first. If you have a MOS-K slide and a K-footprint Holosun, you need no plate from anyone, ours included. If you have the original four-post cut, our AltitudeCraft Optic Adapter Plate for Glock 43X/48 MOS is CNC-machined titanium, 0.058″ thick, and ships with screws and thread-locking compound; comparison against C&H Precision and Forward Controls Design options is in our adapter plate buyer’s guide.

How to Install a Holosun on a Glock 43X MOS

Fifteen minutes, a Torx driver, blue threadlocker and a degreaser. The steps below cover the plate path; on a MOS-K slide, skip step 5.

Tools

• Torx T10 and T15 drivers, or the wrench supplied with the optic
• Blue (medium-strength) threadlocker
• Degreaser or isopropyl alcohol
• Torque driver capable of 10–15 in-lb — recommended, not mandatory

Steps

1. Clear the firearm. Magazine out, slide locked back, chamber checked visually and physically.
2. Remove the slide from the frame.
3. Remove the MOS cover plate — two screws — and set it aside somewhere you will find it again.
4. Identify your cut. With the cover plate off, count the posts. Four means plate; two means direct mount. Do this before you order anything.
5. Fit the adapter plate (original cut only). It should sit flat with no rocking. If it rocks, stop — something is wrong and forcing the screws will not fix it.
6. Degrease the mating surfaces and apply a drop of blue threadlocker to each screw unless yours arrived pre-applied.
7. Seat the optic and hand-thread both screws before tightening either one.
8. Torque to 10–15 in-lb, alternating between screws. Over-torque strips threads; under-torque loses zero.
9. Reassemble and zero at 15–25 yards, then confirm zero after a few magazines.

Can You Co-Witness Iron Sights?

Not with factory Glock sights. They sit too low to appear in the window of any of these optics. If a backup reference matters to you, fit suppressor-height sights and expect a lower-third picture rather than absolute co-witness — the slimline slide simply does not leave the room for it.

Height is also the one real argument against a plate when you have a choice. A plate lifts the optic, and every thousandth you add moves the dot further from the bore and further from your irons. On a MOS-K slide you get the lowest possible mount for free. If you are changing irons at the same time, a sight pusher makes that a bench job rather than a gunsmith job.

Four Mounting Mistakes That Cause Most of the Returns

Common mistake 1 — buying a plate before checking the cut. This is now the single largest avoidable error on this platform, and it did not exist before late 2025. Thirty seconds with the cover plate off saves a return.

Common mistake 2 — buying a full-size Glock MOS plate. The 43X/48 use the slimline pattern. A plate cut for a Glock 17/19/45 MOS will not fit, no matter what the listing photo looks like. Check that the product explicitly says 43X/48.

Mistake 3 — assuming “K footprint” and “RMSc” are the same thing. They are close cousins, not twins, and this article exists because that difference is 0.011″ of interference rather than a rounding error. The Holosun adapter plate comparison walks through the measured geometry.

Mistake 4 — skipping threadlocker, or reaching for the red one. Blue threadlocker is the correct choice; red needs heat to break and turns a battery change into a project. Screws mounted dry loosen within a few hundred rounds.

Frequently Asked Questions

Does the Holosun 407K fit a Glock 43X MOS?

Yes, in both senses that matter — it is the right size for the slide, and it mounts. Whether it needs a plate depends on your cut: original Slimline MOS cut (four posts) needs an adapter plate; MOS-K cut (two posts) takes the 407K directly.

Does the Holosun EPS Carry direct mount to a Glock 43X?

On a MOS-K slide, yes. On the original Slimline MOS cut, no — the EPS Carry is a K-footprint optic and Holosun itself supplies an RMSc-to-K plate for exactly this reason. Note the pistol must be an MOS model either way; a non-MOS 43X has no optic cut at all.

Does the Holosun 407C fit a Glock 43X?

No. The 407C is a full-size housing on the Trijicon RMR footprint and overhangs the slimline slide on both sides. Choose the 407K instead.

How do I know if my Glock 43X MOS is the new K cut?

Check the item number sticker on the pistol case — a MOS-K pistol’s number ends in MOSK. If the box is gone, remove the slide and the MOS cover plate and count the recoil posts inside the cut: two posts means MOS-K, four means the original cut.

Do I need an adapter plate for a Holosun 507K on a 43X MOS?

Only on the original four-post cut. This is a correction to what many guides (including an earlier version of this one) have said: the 43X MOS does not ship with a plate kit, and the 507K is not an RMSc-footprint optic. On the original cut you need a plate; on MOS-K you do not.

What is the difference between the Holosun 507K and 407K on this pistol?

Reticle only. The 507K offers the multi-reticle system — 2 MOA dot, 32 MOA circle, or both. The 407K gives you a single 6 MOA dot. Housing, footprint and mounting behaviour are identical, so pick on how you want the reticle to look.

Can I mill my old slide instead of using a plate?

Yes. Shops including Wager Machine Works and C&H Precision remove the rear posts and shorten the front pair, which converts an original cut into the functional equivalent of MOS-K. It costs more than a plate and it is permanent, but it gives the lowest mount and removes the plate as a failure point.

Will a plate made for the original cut work on a MOS-K slide?

Assume not. A plate designed to clear four posts has geometry the MOS-K slide no longer provides, and several of our returns read exactly like that mismatch — a lip or a protrusion at the rear that stops the plate seating. If you have a MOS-K slide, mount the optic directly.

The Verdict

If your Glock 43X MOS has the MOS-K cut, buy the Holosun you actually want — 407K, 507K or EPS Carry — and mount it straight to the slide. You will get the lowest sight height available on this pistol and one less part in the recoil path.

If you have the original Slimline MOS cut, a K-footprint Holosun needs a plate, and the honest choice is between a good plate now or a permanent mill later. Both work. Neither is wrong. What is wrong is ordering either one before you have taken the cover plate off and counted the posts — that single step would have prevented nearly a third of every return we have on file.

Still deciding on the optic itself rather than the mount? Our Glock 43X red dot comparison covers non-Holosun options, and the fitment and torque FAQ answers the mechanical questions in more depth.

Disclosure: AltitudeCraft manufactures the AC311 adapter plate referenced in this article. Slide-cut findings are sourced to Night Fision, Wager Machine Works, C&H Precision, Primer Peak and Glock retail listings, each linked above; return figures are our own first-party Amazon returns data for the AC311 (n=135, 88 with written comments), current as of August 2026. External vendor links are marked nofollow. Last updated August 2026.

Additional sources: Night Fision — Glock Updates to the Slimline MOS Cut: MOS-K · Wager Machine Works — K footprint direct mount conversion · C&H Precision — 43X/48 slide milling upgrade · Primer Peak — Modifying the Glock 43X/48 MOS for Holosun K optics · SK Guns — Glock 43X MOS “Optic Rdy K Cut” listing (PX435Z201FRMOSK)