In various iterations of Superman's story, Lex Luthor has employed kryptonite in various forms to weaken or kill Superman, and a kryptonite bullet is a common weapon in his arsenal.
However, it's important to note that the specific use of a kryptonite bullet has varied in different versions of the Superman mythos. It's not a universal event, and in some stories, Lex Luthor might have used other forms of kryptonite instead.
Do you have any specific comic books, movies, or TV shows in mind? If so, I can provide more specific information about who used a kryptonite bullet in that particular story.