The Zaffiri Precision RTS Sig P320c ZPS.2 RMR Cut Slide would work with your P320 frame and FCU, however, it is designed for a 3.9" barrel. You would not be able to use your original 3.6" barrel with this slide.
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The Zaffiri Precision RTS Sig P320c ZPS.2 RMR Cut Slide will fit your P320 M18, however, it would not be compatible with the Romeo 1 Pro. This slide uses a Trijicon RMR optic cut while the Romeo 1 Pro uses the Leupold DeltaPoint Pro footprint.
The Zaffiri Precision RTS Sig P320c ZPS.2 RMR Cut Slide will not work with a P320 Compact 40 S&W.
The Zaffiri Precision RTS Sig P320c ZPS.2 features an RMR Cut Slide that will allow for any optic with the Trijicon RMR footprint (like the Holosun 507c) to fit the slide without the need for an adapter plate.
Yes, The Zaffiri Precision RTS Sig P320c ZPS.2 RMR Cut Slide is designed to work with a 3.9" P320 barrel.
The Zaffiri Precision RTS Sig P320c ZPS.2 RMR Cut Slide weighs 6 oz.