Modifying FCD P320 RMR and ACRO plates for P226 and P229

Properly applying RMR-pattern optics on classic Sig P-series pistols. 

Slides

Ensure that your P226/P229 slide has the latest (as of this post) optic cut for direct mounting both the DPP and RMR patterns. 

The older Sig RXP line will NOT work as that was proprietary to Sig's line of optics despite being DPP pattern. I am only aware of the PRO and Legion (shown here) slides having this cut.

Reference photo to ensure you have the proper holes for direct mount of the RMR.

Photo 2 was taken from the Forward Controls Design product page for the OPF-P320, RMR product page.

https://www.forwardcontrolsdesign.com/OPF-P320-RMR_p_449.html

Why do want to use an optic plate? For proper optic support. Photo 3 shows a Holosun 508T mounted but other RMR-patterns will experience the same fitment gaps. The recoil bosses normally present for the RMR are gone, so you are relying purely on the optic screws to take the brunt of the energy. There is also zero support to prevent optic shift, which causes further stress on the screws. Using properly designed plates will engage the support grooves of the slide.

Photos below show what needs to be modified on the FCD OPF-P320, RMR plate. Remove the little fangs. Photo 5 shows where those fangs index on the P320 slide but the P226 and P229 slides do not have that provision.

Optics

You can use a Holosun RMR-pattern optic BUT see my pinned post about countersink angles. The FCD provided screws are specific to Trijicon specs and won't properly seat with Holosuns. The Holosun supplied screws are too short to fully engage here, so you'll have to source your own 6-32 90 degree screws of the appropriate length. 

I have tried the SRO but it sits too far forward and the optic plate raises it enough to cause the malfunction shown in photo 8, so I have RMRs on both now.

The Vortex Defender series is DPP direct mount and might work better here but I have not confirmed how the optic body interfaces with the slide cut.