Bullets bounce off Superman's chest, due to his 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.