Just a guess but it could be that the screws are a fraction too long, so they are bottoming out before they get chance to clamp down the frame.The actual thread of the front frame screw looks in good nick. The head has various nicks and dents, but of course that won't cause the rattling.
What does seem a little disconcerting is that as I'm tightening the allen key in the hole (go on lump, make another innuendo out of that ) it comes to a reasonably abrupt stop, instead of becoming progressively resistant.
-Will
Thats what happened on my ion, took me 2 minutes to file down the screws.
Dam cheap ass guns