Will your iron sights co-witness?With standard-height irons: No co-witness. Standard-height irons sit far below any pistol red dot window and stay usable only with the optic removed. Suppressor-height sights are the fix for that, not a different optic.With suppressor-height irons: Marginal. Suppressor-height irons and an open emitter usually co-witness on a direct cut, but a plate lifts the optic and can push the front sight to the bottom edge of the window or below it. Check the plate's deck height before ordering sights.
How this slide mounts an opticOptic-ready P320 slides are milled for SIG's own Romeo1Pro pattern. Everything else goes on through an adapter, and there is a deep plate market because of it.
What to order, and how to fit it
The fit checker's build sheet for this pair, written for iron sights of unknown
height.
Open it in the
fit checker to set your sights, work out a safe screw length from
your own caliper readings, or copy it as text.
Build sheet
Data compiled September 2026 · Optic Mounting
Pistol
SIG Sauer P320 XCompact
Optic
Holosun 507K X2
Footprint
Holosun K
Slide
SIG P320 optic cut (Romeo1Pro pattern)
Mount
Adapter plate required
Parts to order
1 ×
Holosun 507K X2Holosun K footprint · 2 MOA + 32 MOA circle
$300–450
1 ×
P320 → Holosun K / EPSC&H Precision
$25–60
1 set
Suppressor-height sights — if yours are standardMeasure your front sight first. Under about 0.290 in and it will not co-witness.
Suppressor-height sights — if yours are standardOptional — for a co-witness. Third-party: Trijicon, Ameriglo, XS, Dawson
Fasteners — which bag
You are not buying these. Screws will arrive with the pistol, with the optic and with the plate, and they look alike. The tapped thread is in the slide or the plate — the optic only has a clearance hole — so the host decides the thread, not the optic.
Joint
Thread
Comes from
Plate → slide
set by the slide
Supplied with the plate. If your pistol shipped with a cover plate, its screws are in that same bag and are NOT these. Plate screws and cover-plate screws are different lengths.
Optic → plate
#6-32
Supplied with the plate — not the optic. This is the pair people get wrong. The screws in the optic's box are cut to mount it straight onto a slide. Put a plate in the stack and the geometry changes, so the plate maker supplies the right pair. If your plate arrived without screws, you are measuring, not guessing.
On the bench before you start
Torque wrench — covering 15 in-lb — a 10–50 in-lb driver covers most of this work
Driver bits — T10 — the bit must seat fully or it will round the head
Calipers — for the screw length check below; guessing is how screws end up in the striker channel
Blue removable threadlocker — Vibra-Tite VC-3 or Loctite 242. Never red.
Degreaser — isopropyl or brake cleaner — oil under a plate is why plates move
A parts tray — three bags of screws will arrive and they look alike
Witness-mark paint or lacquer — so later movement is visible before it costs you a zero
Clear the pistol. Drop the magazine, lock the slide back, look and feel the chamber, then do it again.
Remove the factory cover plate and set its screws aside — you will not reuse them for the optic.
Degrease the slide pocket, both faces of the plate and the optic base. Oil under a plate is why plates move.
Seat the plate on the slide, checking it sits flat with no rock, and fit its screws with blue threadlocker.
Torque the plate to the slide to the plate spec, working the screws evenly rather than one at a time.
Seat the optic on the plate's recoil posts, fit the optic screws with blue threadlocker, and torque to the optic spec.
Let the threadlocker cure before firing — check the bottle, usually a few hours to overnight.
Confirm zero at your working distance, then re-check torque after the first 100–200 rounds.
Before you fire it
Screw length checked dry — the screw must not bottom out on the slide or reach the striker channel.
Optic sits flat with no rock, and the screws pulled up evenly.
Torque set with a wrench, not by feel. Witness-mark the screws so movement is visible later.
Threadlocker cured.
Function check, then zero confirmed live.
Screw length is the one that hurts
A screw that is too long bottoms out on the slide or intrudes into the striker channel; one that is too short strips under recoil. This sheet gives you the thread — it cannot give you the length, because that depends on your exact plate and optic stack. Test-fit dry and count threads.
Sight co-witness
Depends on your front sight height, which varies by SKU and production run even within one model name. With standard-height irons: no co-witness. With suppressor-height irons: normally a lower-third co-witness on a direct cut, and the plate in this stack raises the optic further.
Confirm the firearm is unloaded before any work on the slide. Mounting an optic is gunsmithing — work within your competence and use a gunsmith when a job is past it. Provided as-is with no warranty; you use this at your own risk. Prices are approximate street-price bands compiled September 2026, for comparing routes rather than budgeting to the dollar. Torque and fitment figures are compiled from manufacturer documentation — confirm against the current manuals for your pistol, optic and plate before installing. Not affiliated with or endorsed by any manufacturer.
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.