Glock 43X MOS Footprint: RMSc, MOS-K, Screws, and Direct Fit

The phrase “Glock 43X MOS footprint” now has two correct answers. The answer depends on which factory cut is actually on the pistol.

Factory Footprint Matrix

That matrix comes from Glock’s current optic-ready system documentation, not from measuring an anonymous slide photo. Glock says standard Slimline MOS fits the Shield RMSc footprint and uses M4 screws. It says MOS-K is modified for both Holosun K and RMSc, without adapter plates, and uses the optic manufacturer’s M3 screws.

Standard Slimline MOS: RMSc, Not Full-Size MOS

The G43X/48 system introduced in 2020 is not the numbered plate system used by full-size Glock MOS pistols. Glock’s original Slimline MOS announcement calls it a non-standard MOS footprint for micro-reflex sights such as the Shield RMSc.

That means:

• A plate advertised only for “Glock MOS” is not automatically a G43X/48 plate.
• Full-size RMR-pattern optics are not factory direct fits.
• A genuine RMSc optic is the native pattern.
• Holosun 407K/507K fit advice must account for the recoil-post difference unless the pistol is MOS-K.

Use the Fit Checker with standard Slimline MOS selected to inspect individual optics.

MOS-K: A Dual-Pattern Cut

MOS-K changes the interface so the same slide cut accepts both the Holosun K and RMSc families. Glock also changes the screw system to M3 optic-maker hardware. The official MOS-K accompanying sheet warns owners to use the optic manufacturer’s installation information and periodically check secure attachment.

The crucial naming clue is the K in the item number, not the word MOS by itself. Retail listings may shorten names, inventory photos may be reused, and the slide view is easy to misread. Ask for a photo of the case-label item number when buying remotely.

RMSc and Holosun K Are Related, Not Identical

Both families place two screws in a similar narrow-sight layout. The K pattern removes the rear recoil sockets and uses shallower front socket geometry. A standard four-post RMSc cut can therefore block a K optic from seating even when the screw holes appear aligned.

“The screws line up” is not a fit test. A proper direct fit requires:

• the optic base to sit fully flat on the cut;
• recoil features to mate without interference;
• correct thread, length, and engagement;
• no screw contact with internal slide parts; and
• the maker’s specified torque and retention procedure.

On a standard G43X MOS, bridge K optics with an exact plate. On MOS-K, Glock has already changed the cut for direct K compatibility.

Verified Direct-Fit Examples

These are examples, not a promise that every product marketed as “RMSc compatible” uses identical hardware.

The strongest model-specific direct-fit statement belongs to the SIG ROMEO-X Compact: SIG explicitly lists Glock G43X and G48 and provides both M4 and M3 hardware families. Vortex publishes the Defender-CCW product and footprint resources, but the final supplied screw choice still needs checking against the exact Glock cut.

Screw Rules Without Guesswork

Footprint answers “where are the attachment features?” It does not answer every hardware question.

• Standard Slimline MOS: Glock says M4.
• MOS-K: Glock says use the optic maker’s M3 screws.
• Glock’s cover screws hold the cover only; do not reuse them for an optic.
• A torque printed for a cover plate is not automatically the optic’s torque.
• A plate may use one screw set into the slide and another into the optic; follow the exact plate instructions.

Dry-fit before thread treatment and torque. If the optic or plate rocks, gaps, or stops on a screw before clamping, stop. Do not use final torque to pull mismatched geometry together.

What to Buy After Identifying the Cut

• Standard MOS + RMSc optic: buy the optic and confirm its listed M4 hardware for G43X/48.
• Standard MOS + 507K/407K: add a named G43X/48 MOS K-footprint plate.
• Standard MOS + EPS Carry: add a named G43X/48 MOS EPS plate.
• MOS-K + RMSc or K optic: no adapter by Glock’s published interface; use the optic maker’s M3 screws.
• Non-MOS G43X: choose professional milling or a replacement slide, then follow the new cut maker’s documentation—not this factory table.

For product selection rather than geometry, return to the G43X MOS red-dot buying guide. For the broader interface distinction, see RMR vs. RMSc footprints.