I just took my Mark III to the range today with the kids (who had a great time, BTW), with a Tasco red dot on a Jack Weigand rail. I had cleaned the screws and holes before assembling it dry. After about 150 rounds, the rail was very wobbly, as the screws were backing out, so I removed the sight and the rail for the rest of the day. (Another 200-300 rounds with the iron sights.)
I think I'll get some blue Loctite...
Dan