>>34343149Boobs are mostly fat, and fat doesn't slow bullets down much. However, sagging boobs could mean two extra layers of skin in the way of the bullet, and skin slows bullets a fair bit.
It's definitely plausible, in some rare edge cases where otherwise a bullet would just barely reach vitals, (example: a shotgun loaded with ammo meant for hunting little birds) that a boob could make enough of a difference and save a woman.