Bullets bounce off Superman's chest, due to his invulnerability.
Invulnerability is the ability to be immune or highly resistant to one or more forms of physical damage, injury or disease and illness. Both Kryptonians and Daxamites have this power, among others, under the effects of the yellow sun. Some metahumans and other superbeings also have varying degrees of invulnerability.
Kryptonians and Daxamites seem to gain this ability as a result of the Photonucleic Effect, that is, exposure to Earth's yellow sun. However, it is possible that they may have some degree of invulnerability even without the sun's effects; For example, Superman seems to have been operating largely without sun-granted powers in early issues of Action Comics, and was still able to withstand anything less than a bursting shell against his skin.