Apex Optic Mount Holoson 509T to SIG P320 with SIG-LOC Optic Cut

This Apex Optic Mount adapts the Holosun 509T footprint to any SIG Sauer P320 slide with the factory SIG-LOC optic cut. The primary problem it solves is optic fouling from blowback gases and debris exiting the ejection port. By positioning the rear sight forward of the optic, the sight base functions as a gas shield, deflecting carbon and particulate away from the objective lens. This extends cleaning intervals and maintains a clear sight picture under sustained fire.

Compatibility

• Sig Sauer P320 Legion (full-size, carry, X-Five)
• Sig Sauer P320 M17 / M18 (factory Loaded Chamber Indicator compatible)
• Any P320 slide with factory SIG-LOC optic cut
• Holosun 509T (all generations)
• Does NOT fit standard RMR or Deltapoint Pro cuts

Key Features

• Machined from billet steel with black oxide finish – matches factory slide wear
• Integrated rear sight sits forward of the optic, acting as a gas shield
• Rear sight co-witnesses in the bottom 25% of the Holosun 509T window
• Tall front sight included with each kit – two height options based on barrel length
• Pre-applied thread-locking patch on screws – torque to 20 in-lb, no additional loctite required
• Sights regulated for POA=POI at 10 yards with 115 Gr. ammunition
• Works with factory Loaded Chamber Indicator on M17/M18 slides
• Rear-sight-behind-optic version includes a blast deflector (not visible through the optic window)

Install Difficulty: 3 / 5

Requires removal of the factory optic cover plate (if present), screw installation with torque driver set to 20 in-lb, and front sight installation. Tools needed: #6 hex driver, torque wrench (inch-lb), front sight pusher or punch/brass hammer. Recommended for experienced armorers or those comfortable with slide work.

FAQ

Will this mount work with my P320 X-Compact with SIG-LOC cut?

Yes, provided the slide has the factory SIG-LOC optic cut. The mount will fit compact and carry-length slides. Select the tall front sight option matching your barrel length for proper co-witness.

Do I need to remove the rear sight from the slide?

No. This mount replaces the optic cover plate only. The integrated rear sight is part of the mount, not the slide. The factory rear sight must be removed only if you are using the rear-sight-behind-optic version (Apex 112-018-PRO).

Why does the rear sight sit in front of the optic?

The rear sight base doubles as a gas shield to deflect blowback gases and debris away from the objective lens. This reduces carbon buildup on the glass, keeping the optic clearer during extended strings of fire. The co-witness position is calibrated for the bottom 25% of the Holosun 509T window.

Shipping & Warranty: Ships within 1–2 business days. This component is covered by Apex Tactical’s limited lifetime warranty against manufacturing defects. No returns on installed optics mounts.