BRD makes a quality fixture that uses the indexing pin of the barrel to align the FSB against, which should help you perfectly center over the gas port.

Rumor is Colt would install barrels into the uppers first, then index of the carry handle to install the FSB... which means the sight isn't canted in the least, but also that minor clocking in the receiver means the gas port isn't necessarily perfectly centered under the aperture in the FSB. On a carbine gas lengths system with .062-.070" ports you have plenty of wiggle room... but on .093/.0995" sized rifle length ports, I'd rather index off the gas port/ indexing pin than off the rear sight.