Since the P320 is the civilian version of the M-17, I looked up what the military uses on their M-17s.
I found a thread on sigtalk indicating SOCOM selected this:
I've also seen some branches are using actual Sig optics.
SIG SAUER Electro-Optics is proud to introduce the ROMEO-M17 pistol mounted red dot sight. Developed from the ground up for the U.S. Army’s Modular Handgun System to be the ultimate in performance, reliability and durability.
www.sigsauer.com
^^ states "The ROMEO-M17 is designed specifically to fit U.S. Military Modular Handgun System M17/M18 pistols, M17/M18 Commemorative Special Editions and M17/M18 Surplus pistols with a single hole in the
slide to secure the sight plate. It does not fit standard P320 or commercial P320-M17/M18 slides. Commercial variations of compatible pistols and slides will be available in the coming months. "