how come superman can stop bullets with his chest but he always ducks when someone throws a gun at him??/

Lol. OP came to the platform for only ten days and ran away. As usual he didn't post examples as a form of proof. Let me answer the question using the Man Of Steel Superman:

Clark Kent grew on earth. This means just like regular humans he knows guns are meant to kill. As such he fears them. If you notice throughout the MoS till Justice League films he avoids coming into contact with firepower of any sort. This is because he doesn't know the extent to which his powers work. When shot with a pistol he flinches at first till he realizes the first two or three shots don't penetrate his skin. After those he releaxes and does his thing as Superman. From there we climb through the heirarchy of projectiles, with him gaining confidence against them. His first reaction as always is to avoid direct contact of any sort: self-preservation from living on earth for thirty years. By the way, a thrown gun also hurts, so he too will avoid it on instinct.