Will an AR-15 upper fit an AR-10 lower?

An AR-10 lower is built around a larger cartridge, so the receiver itself is bigger — the takedown pin spacing, the magazine well and the receiver face are all different. An AR-15 upper will not mate to it, and the resemblance between the two platforms is the reason people ask.

The more expensive trap is inside the AR-10 world rather than between the platforms. The AR-15 has a mil-spec pattern most manufacturers follow; the large-frame rifle never got that, because it was commercialised independently by more than one company. The result is two families that do not interchange: the DPMS or LR-308 pattern, and the Armalite pattern. An upper from one will not fit a lower from the other.

The rear of the upper where it meets the lower is the quickest tell: DPMS-pattern receivers use an elliptical cut and Armalite-pattern an angular one. If you are buying parts for a large-frame build rather than matching an existing rifle, DPMS is the pragmatic choice simply because far more parts are made for it.