When I was still physically able able to shoot service rifle category before I became disabled, I acquired one of Bob's lenses for both my AR and M1A. They slide into the rear sight hood and are held in place by an "O" ring. The nice thing about his lenses are that the rifle can still be shot as a service rifle, and you can still use your glasses. They are not a replacement for your glasses.
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Master NRA High Power, Sharpshooter NRA Long Range, Marksman NRA Mid Range