He would have to use the adaptor that Sig supplies with all uppers it sells. The Sig adaptor adds the same pic rail to the rear and also has supports on the shoulders for the recoil springs. Those supports prevent an MCX lower from working well with some of the other bufferless uppers such as the BRN-180. An MCX lower sold separately is uniquely useful to someone that has an MCX upper without buying a complete MCX and is not satisfied with the supplied adaptor. Considering the incredibly small amount of uppers shipped by Sig vis-à-vis complete guns, the overlap of those two groups is likely incredibly small. For some reason OP is convinced there is a market for MCX specific lowers.