NavList:

And in the world of M60 tanks, we called these "testing targets." The M32 daylight sight is located near the top of the turret, more than a meter from the centerline of the 105mm main gun tube. The M109 direct  fire scope is about 50 cm to the right of the gun. In order to hit an enemy we have to verify that the crosshairs in the sights are exactly aligned with the line of the gun bore to eliminate parallax . So we stretch two shoelaces across the end of the gun barrel at four witness marks, horizontal and vertical, this makes a set of crosshairs at the muzzle of the barrel. Then we open the breach and put in a gauge that puts a peephole at the very center of the bore at the breach end. Looking through this peephole and lining up with the crosshairs at the muzzle establishes the line that the shell will leave the gun at. We then put out the testing target 50 meters to the front. The testing target has aiming points located at exactly  the same distance from the boreline as the lenses of the fire control equipment. So we move the gun til the line established down the bore is exactly aimed at the mark representing the boreline on the testing target. Then we look through the daylight sight and the M109 and make adjustments to their mounting adjusters until they are aligned with the appropriate aiming points on the testing targets. 

So, it should work with the sextant.