The reason for having this is to protect your red dot from bb's. I got this in the mall yesterday and i posted it up on my AK. I accidentally (and lightly) bumped it into a wall in my house and the screw holding the protector flew out and the lens it self chipped. Concerned, I decided to do a couple of test shots with a spring powered pistol from 10 ft. away. IT SHATTERED! That pistol only shoot 120 FPS!!!!! now the screw that flew out wont even screw back into the mount.
The field I play at has an FPS limit of 330. Doubt this will do anything to protect your sight. And if it does you'll be replacing it every time it gets shot at.