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

What This Mount Does & Why You Want It

If you’ve got a Sig P320 with the newer SIG-LOC optic cut and want to run a Holosun 509T, this mount is the missing link. It’s a machined steel plate that replaces the factory optics cover, giving you a rock-solid mounting surface for the 509T. The big deal here is the integrated rear sight—it sits in front of the optic, acting as a gas shield to keep carbon and debris off your lens. That means less wiping, clearer glass, and one less thing to worry about at the range. It also saves you the hassle of having to mill a separate dovetail for a backup rear sight.

Will This Fit My Gun?

This mount is designed specifically for Sig Sauer P320 slides with the SIG-LOC optic cut. That includes:

• P320 Legion (all variants with SIG-LOC cut)
• P320 M17 / M18 (works with factory loaded chamber indicator)
• Other P320 models factory-cut for SIG-LOC (not the older “Delta Point Pro” footprint)

Optic compatibility: Holosun 509T (any generation) — mounts directly, no adapter plate needed.

Key Features

• Machined from billet steel with a black oxide finish — tough, corrosion-resistant, and matches the factory slide look.
• Integrated rear sight acts as a gas shield — sits in front of the optic to block blowback debris from the ejection port. Keeps your lens cleaner longer.
• Co-witness setup: Rear sight is calibrated for the bottom 25% of the Holosun window. Tall front sight included with your mount choice.
• Two sight height options — select based on your barrel length (see dropdown above). Both regulated for POA=POI at 10 yards with 115-gr ammo.
• Thread-locking patch pre-applied to the mounting screws. Torque to 20 in-lb — no thread locker needed.
• Rear-sight-behind-optic version also available (includes a blast deflector not visible through the glass).

Install Difficulty: 3 out of 5

Tools needed: Small flathead screwdriver (for rear sight removal if replacing an existing plate), T10 or T15 Torx driver (for the optic screws), and a torque wrench (inch-pounds). If you’re swapping from a factory cover plate, it’s straightforward — remove the cover, drop in the mount, torque screws to 20 in-lb. If you’re also replacing a rear sight or need to remove an existing optic, budget about 30 minutes. We recommend watching our install video on the product page before starting.

Short FAQ

Will this work on my P320 X-Five or X-Carry?

Only if your slide has the SIG-LOC optic cut. Earlier P320s (pre-2022) used the “Delta Point Pro” footprint. Check the underside of your slide — if you see two small alignment pins and a rectangular cutout, it’s SIG-LOC. If it’s a single large square cutout, it’s DPP and this mount won’t fit.

Does the rear sight hold zero?

Yes. The sight is machined as part of the mount, not a separate pinned piece. Once the mount is torqued to 20 in-lb on the slide, it’s solid. The sight is regulated for 10-yard POA/POI with standard 115-gr loads.

Can I use my factory front sight