The C79 has its own unique design. The optics are sealed. Not adjustable for vision. All adjustments for windage and elevation are controlled by the interaction of the optical portion and the base. The optics are spring loaded into place against the bearing surfaces of the base. The tube is shifted mechanically to change point of impact. You can grab the tube and move it against the spring. If the tube shifts ever so slightly against the base - if it doesn't return to exactly the same position after each shot, zero will change.
In the case of the C9, the tube bounces on its base, and the top cover moves on the gun. Hardly a stable sighting arrangement.
I believe the design is now on its 4th generation of base design.
So, will it hold zero on a 7.62?