Holosun 507K on a Glock 43X MOS: Plate or Direct Fit?
The Holosun 507K fits a Glock 43X MOS, but standard MOS and MOS-K use different mounting paths. Here is the verified plate and screw answer.
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A Holosun 507K fits a Glock 43X, but not by one universal route. On the original standard Slimline MOS cut, use an adapter plate expressly made for the G43X/48 MOS and Holosun 407K/507K. On a Glock 43X MOS-K, the 507K mounts directly because Glock modified that cut for the Holosun K footprint; use the optic maker's M3 screws. Do not reuse the cover screws or force the 507K over the standard cut's RMSc recoil posts.
The Holosun 507K is a valid G43X option. The phrase “direct fit” is valid only after identifying which G43X factory cut you own.
Fit Verdict
| Glock configuration | 507K mounting path | Adapter? | Screw rule |
|---|---|---|---|
| G43X standard Slimline MOS | Exact G43X/48-MOS-to-407K/507K plate | Yes | Follow plate maker; standard cut is M4 |
| G43X MOS-K | Direct to factory cut | No | Optic maker’s M3 screws |
| Non-MOS G43X | No factory mounting path | N/A | Professional cut or optic-ready slide |
Glock’s optic mounting page defines standard Slimline MOS as Shield RMSc with M4 screws and MOS-K as a modified RMSc/K cut using optic-supplied M3 screws. Holosun’s 507K X2 page identifies the sight as a K-footprint optic and lists both M3 and M4 screw sets in the box. Those facts establish the interface family, but the slide cut still determines which set and whether a plate belongs between them.
Why Standard MOS Needs a Plate
The Holosun K footprint is derived from Shield RMSc but does not retain the same recoil-socket layout. The standard Slimline MOS cut keeps the RMSc post geometry. A 507K can therefore align over the screw holes yet fail to seat flat because its base does not accept all four RMSc posts.
An adapter provides K geometry on top and the correct standard MOS interface underneath. It also supplies a controlled hardware path. The C&H G43X/48 MOS 407K/507K plate is one manufacturer-documented example: its listing names the two pistols and both optics and specifies its included M4 plate screws.
That exact naming matters. “Glock MOS to Holosun” can describe a plate for a full-width Glock and a 507C, neither of which answers this fit.
Why MOS-K Does Not Need a Plate
Glock engineered MOS-K to accept the K base directly while keeping RMSc compatibility. Glock says to use M3 optic screws provided by the optic maker and makes clear that the cover screws are not optic hardware. The MOS-K instruction sheet also directs owners to the optical-sight manufacturer’s instructions.
The 507K box currently lists M3x12 and M4x12 screws. Do not choose by thread alone: confirm the exact screw length and current instruction for the MOS-K slide. A screw can have the right thread and still be too long.
407K vs. 507K Does Not Change Fit
The 407K X2 and 507K X2 share the K mounting family. The purchasing difference is primarily the reticle system: the 507K offers a selectable 2-MOA dot, 32-MOA circle, or combined display; the 407K is the simpler single-dot option. The standard-MOS plate route and MOS-K direct route remain the same when the plate listing names both models.
Installation Sequence
- Unload and clear the pistol, then remove the slide following Glock’s manual.
- Confirm the case item number says MOS or MOSK.
- Standard MOS: dry-fit the exact plate to the slide, then the 507K to the plate. MOS-K: dry-fit the 507K directly.
- Confirm every surface seats flat and each specified screw clamps before bottoming out.
- Degrease and apply only the thread treatment specified by the optic or plate maker.
- Torque in the sequence and to the value supplied for that exact interface—not a value copied from the cover plate.
- Function-check the unloaded pistol, witness-mark the fasteners, zero, and inspect them on the maker’s recommended schedule.
If the plate or optic rocks before torque, or the slide stops functioning normally afterward, stop and correct the hardware path before live fire.
Plate vs. Permanent Modification
Some owners remove recoil posts from a standard Slimline MOS slide to create K clearance. That can produce a lower optic position, but it is irreversible, changes the factory interface, and makes the result dependent on the machinist’s dimensions and screw plan. It should not be presented as “direct fit.”
Use a documented adapter for a reversible installation. If low height is important enough to justify milling, choose a qualified shop that warrants the finished K cut and tells you exactly which hardware to use.
Bottom Line
- Case says MOS: buy a 507K and an exact G43X/48 MOS-to-407K/507K plate.
- Case says MOSK: the 507K is a factory-documented direct-fit footprint; use the specified M3 optic screws.
- No factory cover: stop applying MOS instructions; the slide needs a different mounting solution.
See the broader G43X MOS red-dot guide for alternatives, or open this exact pairing in the Fit Checker.
Frequently Asked Questions
Does the Holosun 507K fit directly on a Glock 43X MOS?
Only if the pistol is the newer MOS-K variant. Standard Slimline MOS is an RMSc/M4 cut and needs a G43X/48-MOS-to-407K/507K plate. MOS-K directly accepts the K footprint with optic-supplied M3 screws.
What adapter plate fits a Holosun 507K on a Glock 43X MOS?
Use a plate whose manufacturer names both the G43X/48 MOS and the 407K/507K. C&H's GLX-HOLOk is one documented example and includes its specified M4 plate hardware. Do not buy a generic full-size Glock MOS plate.
Can I remove the rear posts instead of using a 507K plate?
That is a permanent slide modification, not Glock's documented path for standard Slimline MOS. A model-specific adapter is the reversible route. If a direct mill is desired, use a qualified shop that specifies the finished interface and hardware.
Do the Holosun 407K and 507K use the same Glock 43X plate?
Yes when the plate maker explicitly lists both. Holosun puts both optics in the K mounting family, and C&H's G43X/48 MOS plate names the 407K and 507K together.