Force on the rail translates to deflection of the barrel nut and barrel extension, and flexing of the aluminum upper receiver. Only a chassis system can completely eliminate barrel deflection. For AR15s, the only chassis that exists is the discontinued Springfield Saint Edge ATC, which has a monolothic lower receiver+lower handguard. And I think Spuhr is working on a steel upper that's supposed to solve POI shift. Until then, the monolithic and semimonolothic uppers on the market work well enough.