Possibly a reference to the fictional 'vampires can only be killed if they're shot by a silver bullet' as normal lead bullets presumably aren't effective. Like

Possibly a reference to the fictional 'vampires can only be killed if they're shot by a silver bullet' as normal lead bullets presumably aren't effective. Likewise, it must be a 'wooden' stake through the heart. Etc, etc. More or less it implies that there is only one effective solution to a problem, and that is what must be used if you want the problem to go away.