Don't say "sorry," Murphy -- you've added greatly to the discussion. We learned the trick years ago, and were told to use a dot about 1/2". It was clearly annoying so we evolved first to Magic Tape, then to a small grease smear. It has seemed to work well.

Your point of obscuring the muzzle end (only) of the gun from the off-eye's perspective creates an "ah-ha!" moment. I have no doubt it will work better, thank you!

Fred