How come Superman could stop bullets with his chest but always ducked when someone threw a gun at him?

It might be due to a lack of funding for quality, and lack of ability in some cases, for special effects. The Man of Steel was concerned for the by-standers who might get shot if the gun mis-fired when it bounced off of him. IT is easy for an actor to Dodge a prentend bullet than it is to dodge a real hunk of iron.