by Fred Guterl

About the author: Fred Guterl, staff writer for Discover, writes about topics in science and medicine.

Sam Walker was not your average American gun owner. For one thing, he had no interest whatsoever in hunting. And whereas the average gun owner owns at least three guns, Walker owned only one, a .38-caliber revolver, which friends persuaded him to buy for the sole purpose of protecting himself and his family in their suburban Houston home. Walker didn’t even particularly like guns. He still hadn’t gotten around to acquainting himself with his new weapon when his burglar alarm went off one weekday morning in December 1996. Notified by his security company of the intrusion, Walker rushed home from work, quietly entered the house, took the gun out from the spot...