Scientific American's partial answer:
The bullet, reportedly fired from a 9 mm semi-automatic handgun, entered the left side of the back of her head and traveled through her brain, exiting through her forehead. This “through and through” course indicates that the bullet was probably not of the variety designed to expand or break into smaller fragments once inside the body, which would have caused a much more severe injury. Further, the Congresswoman would have sustained much more damage had the bullet ricocheted off the inside of her skull, or if it had remained lodged in the brain matter where it could “migrate around” and cause additional harm, says [Dr. Keith L. Black].
(Photo of Giffords' husband, Mark Kelly, by her bedside, via TDW)