I have been using an Optivisor for thirty years. They have glass lenses, keeps the scratches to a minimum. I use a 5 power, it has a short working distance, which is ok for detail work. I use them for my knife work, don't know what working distance is needed for gun work. Their website lists the power/working distances. One problem I have found with the new Optivisors are the plastic visor bolts they have gone to. They are impossible to adjust for proper tension, keep coming loose. That feature would make me look for a different manufacturer when mine need replacing.
Wolfgang