Some reviewers might be wrong - Meprolight M16/AR15/A1 Front Sight

As pictured, the tight has an upper part which rotates on the lower part, which has the threaded screw by which it is threaded into the Front sight base. IOW , the upper part, containing the Tritium-illuminated post rotates independently of the base.

For sighting-in, simply dial-in the sight, as a whole, at desired distance. Once that has been accomplished, then carefully depress locking pin [I]just[/I] enough to turn the upper front sight post so that the Trit element (top post) is visible to the shooter, [I]without[/I] turning/disturbing the lower part of the assembly. Turning the independently rotating top post of the sight, without disturbing the bottom of the sight will not disturb the sighting-in calibration, and will allow the user to see the Tritium night sight.pm This is not easily done, but it's quite possible for most users to do.