A boresight alignment instrument having a helium-neon gas laser mounted on an alignment mandrel which fits into the gun barrel. A 45.degree. mirror directs the laser beam along a line concentric with the axis of the bore of the barrel. The mandrel has an expanding collet to insure positive locking of the mandrel in the barrel. The laser beam is aligned with the desired output axis, of the adapter assembly, by securing the adapter assembly and laser to an alignment fixture which permits rotation of the laser and adapter assembly. The mirror position is adjusted to stop all motion of the light spot when the assembly is rotated 360.degree.. The device is used with a highly reflective target to make the device useful under all lighting conditions.