NavList:

Gary LaPook, you wrote:
"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."

Very practical! But I must ask, were these actual shoelaces, pulled right out of your own shoes? Or did they come in a special pouch designating them as "crosshair laces; only for use in gunsight alignment tests"? Heh. :)

You continued:
"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."

That's great! Thanks for all that detail. Without a diagram, I'm not quite sure I followed it all, but it does sound similar to what we could do for a sextant test. In fact, it sounds even closer to a test that I used to recommend for sextant "scope collimation".

The telescope on an older sextant can easily be tilted at an annoying angle, thanks to a mis-shapen mounting yoke usually. This is rare on new sextants. If there is significant collimation error (scope tilt), we can use a carpenter's laser-level resting on the frame (right on the arc). The bright "dot" of the laser will create a relatively distant sighting target that has a height above the plane of the sextant that is known, measurable and dependable, since the beam is parallel to the sextant frame. This height can, with a little effort, be made identical to the height of the telecope above the frame. Then when we look through the scope, that dot should be exactly centered, at least in the vertical direction, in the field of view or on the crosshairs (for scopes that have them). If it's not, with any luck we can adjust up or down to get the scope properly aligned with the frame and optical plane of the sextant. This works. And I "think" it's similar in principle to the method you used in that tank years ago. :)

Frank Reed