Abstract: The disclosure relates to a boresight unit wherein an electro-optic package is disposed in a gun shell casing and loaded into the breach end of the main gun tube of a tank for use in boresighting visible or thermal gunsights. The package includes a laser aligned with the axis of the shell and main gun tube as well as an optical system for controlling the size of the light beam emanating from the laser. The gun is aligned by placing the boresight unit in the gun breech, making the appropriate adjustments to the gun and then replacing the boresight unit with a live shell.