The figurative usage of "silver bullet" isn't necessarily as old as "magic bullet," but it goes back at least to the 1950s. Plus, "magic bullet" usually has a m

The figurative usage of "silver bullet" isn't necessarily as old as "magic bullet," but it goes back at least to the 1950s. Plus, "magic bullet" usually has a medical connotation owing to the early use of the term by Paul Ehrlich. More importantly, though their range of effectiveness seems to have expanded over the years, silver bullets are traditionally used against werewolves, not vampires.