Symbolism, Imagery, Allegory
How much can one little silver bullet mean? Lots, apparently. When Sanford picks it up at the scene of Casimir's death, everyone's too spooked to give it much thought. But then Horace asserts: "This slayer's misinformed. He thinks a silver bullet is going to do more damage than a lead one" (4.3). The fact that Casimir's slayer doesn't know that silver isn't full of anti-vampire magic means that he's not a true expert on all things vampire, which works in everyone's favor.
How so? Well, besides the obvious generalization that it's better to be hunted by someone with bad info than someone who's truly on top of things, to be specific, their slayer's bad into means the vampires can hole up at Nina's mom's house because this slayer won't believe that vampires can peacefully coexist with a human. The little silver bullet is more than just a clue to the slayer's inner workings, however—it's also a ticket to the illegal werewolf fighting ring.
When Nina, Dave, and Father Ramon trek out to Cobar, having gotten a list of customers from the manufacturer of the silver bullet, they meet Dermid, who's collecting silver bullets for entry. But seeing a priest and two pale people walk up isn't what Dermid expects: "Something clearly didn't make sense to Dermid. Our silver bullet had thrown him" (10.47). They might look off, but they have the ticket to entry—a.k.a. a silver bullet—so Dermid ignores their appearances and lets them on in.
This exchange shows that the silver bullet has multiple meanings. To Dermid, it's a pricey ticket to attend the bloody spectacle; to our fearless gang, it's a clue connected to Casimir's death; to the unnamed slayer (who we later find out is Nefley), it's one more arsenal in the war against the supernatural baddies. If this isn't an example of how people can imbue an object with darn near any meaning, we don't know what is.