YoungestPiperCub is still incorrect. The proprietary Receiver contours used on the BR10 series of rifles will not interchange nor mate up with àny modern AR10 Receivers, neither the Lower nor the Upper. ,,,,
Proof.
The first pic compares the brownies upper (top) with a Sudanese AR10 upper (bottom). See the contour is similar but the locking hole is way off. A later Portuguese upper would differ from the Sudanese mostly in the diameter of the pivot hole. So brownies upper/lower does not fit the original Dutch locking and pivot pin holes. Note that Hollywood and Sudanese uppers and lowers can interchange so the incompatibility applies to ALL early (original) AR10s.

The second pic shows and attempted mating of a brownies upper with an original Dutch (Portuguese contract) lower. See the minor gap in the profile near the locking pin hole? And the complete misalignment of the locking holes? And the off-size and off-center issues with the pivot pin holes?
Then there is the incompatibility of the skinny modern bolt (brownies) go back into the cavernous original buffer tube...
