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Shield RMSc vs Holosun K: what’s the difference?

The screw spacing is shared; the recoil posts and, usually, the screws are not. A Holosun K optic drops onto an RMSc cut and leaves the rear posts empty. An RMSc optic on a slide cut for K only has no rear posts to index on, and doesn’t fit.

Shield RMSc

front ↑
Screws
M4 × 0.7 (some hosts #6-32)
Torque
10–15 in-lb per plate maker
Interface
2 screws, 4 recoil lugs (2 front, 2 rear)
Pattern
≈0.658 in screw centers

Holosun K

front ↑
Screws
#6-32
Torque
15 in-lb
Interface
2 screws, 2 forward lugs (rear posts omitted)
Pattern
shares RMSc screw centers

Diagrams are schematic — top-down, optic front at the top — and not to scale.

Which goes on which

OpticOn a slideAnswer
Holosun 507K X2
Holosun K		GLOCK 43X MOS
GLOCK Slimline MOS (43X / 48) — cut for RMSc		Direct · alt
Direct mount — cross-compatible
Shield Sights RMSc
Shield RMSc		GLOCK 43X MOS
GLOCK Slimline MOS (43X / 48) — cut for RMSc		Direct
Direct mount
Holosun 507K X2
Holosun K		CZ P-09 Nocturne / P-09C Nocturne
CZ Nocturne direct cut — cut for K only		Direct
Direct mount
Shield Sights RMSc
Shield RMSc		CZ P-09 Nocturne / P-09C Nocturne
CZ Nocturne direct cut — cut for K only		No fit
No known fit

Each row is the fit checker’s own verdict for that pair, so it says what every page on this site says.

The screws are the trap

The thread is tapped into the slide, not the optic — the optic only has a clearance hole. So a K optic on an RMSc-pattern slide takes the slide’s screws, not the ones in the Holosun box. The data here has the Shield RMSc pattern as M4 × 0.7 (some hosts #6-32) and the Holosun K as #6-32; your pistol’s page gives the slide’s own thread where the maker taps it.

GLOCK Slimline MOS (43X / 48)GLOCK taps the Slimline MOS slide M4 × 0.7. A Holosun K optic will bolt on, but the #6-32 screws in the Holosun box will not thread into it — you need M4.
GLOCK Slimline MOS-KTapped M3, not the M4 of the earlier Slimline MOS. The two slim GLOCK cuts look alike and take different screws — this is the single most common cross-threading mistake on slim GLOCKs.

Shield RMSc optics tracked here

Holosun K optics tracked here

Every slide cut for Shield RMSc → · every slide cut for Holosun K →

For your pistol and optic

Pick your pistol and optic for the verdict and the slide’s own screw thread.

Something on this page wrong? Say what it should be. Corrections from someone who owns the pistol are why this is worth more than a spec sheet — and nothing here changes without a source, so bring one.