The Remington 3200 O/U shotgun was designed so that it could be safely dry fired. The firing pins ride forward when the action is closed and places them out of reach of the hammers. When a shell is in the chamber, firing pins are pushed back within striking distance of the hammers, which each have their own stops to limit forward travel. It's a neat setup, but then again, I am a big 3200 fan, so I think most things about 3200 are just plain neat.

The way I understand it, dry firing older production guns was harmful because of the metals used in their firing pins. When you dry fired these guns, the persistant hammering of the firing pins work-harden the steel, making them brittle. I have been told by several gunsmiths that today's metallurgy is much better, and dry firing recent production guns poses little harm to their firing pins. Rimfires are an exception, of-course, because you run the risk of peening the edge of the chamber. I don't dry fire my firearms very often, anyway, so it's not something I ever worry about.