I have all the P320’s (I know. I could have just bought 1 and swapped the Fire Control Unit into another frame). The 9, 40 & 357 Sig‘s FCU can be swapped between calibers listed, while the 45 fcu cannot be swapped with the 9, 40, & 357 Sig, only with its own grips, slides & barrels. I bought a Xchange kit to make my 40 compact into the subcompact, an Xchange kit for my 9mm subcompact into the compact, several small grips for the 45, (making my 45 carry into a subcompact), and assorted grips for the 9,40,357 with the different size Mags to fit. It‘s a fantastic system to allow you to swap & build different size & caliber pistols. But be aware that the Xchange kits cost about $100 less than if you just bought a new P320 (at least when I bought them). BTW, I found the Website “Proven Arms Outfitters (formerly Quantico Tactical)” to have very good prices on grips, Xchange kits & Mags for the P320 (when in. Stock). And if you happen to be a First Responder, Veteran, or LEO active or retired, (which I’m not), good prices on the pistols themselves.