BudsGunShop.com - P320 M18

Sam, it will fit with Zero issues Sir. I have been using one for 6 months now and it has performed great so far. cheers and stay safe!

Yes it'll fit but honestly, for the same price you, can get a Holosun 507C or 508T which are in my opinion ( and the opinion of most who've used them) much nicer, they have more/better features ( 50,000 hour battery, solar backup, multi reticle options, and are way more durable)
Some of the sig rifle optics are good (romeo5, and romeo 4. As well as some of their LPVO's) but most are unhappy with the pistol dots they sell.
in stock here
kenziesoptics.com/product-category/red-dot-sights/holosun/
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YES, according to Sig "The ROMEO1 is meant to mount directly to SIG 320RX pistols, as well as to many other popular pistols on the market, by utilizing our available Handgun Adapter" More details HERE ( sigsauer.com/romeo1-1x30mm.html ) I hope this helps!