Vulnerable to Bullets

"...a sacred bullet fired into the coffin kill him so he be true dead."
- Professor Van Helsing in Mina Harker's Journal, Chapter XVIII of Dracula by Bram Stoker.

It seems most likely that Stoker also got the general notion here from Emily Gerard. In "Transylvanian Superstions", she wrote:

"....every person killed by a nosferatu likewise becomes a vampire after death, and will continue to suck the blood of innocent people till the spirit has been exorcized, either by opening the grave of the person suspected and driving a stake through the corpse, or firing a pistol shot into the coffin."

 In the archives of the University of California at Berkeley, there is an account from a Serbian immigrant who said that a silver coin with a cross on it could be broken into four pieces, loaded into a shot gun, and fired to kill a vampire.It can be found in Vampires, Burial, and Death: Folklore and Reality by Paul Barber, footnote on p. 54

In Eastern and Central Europe, people would sometimes fire rifles or pistols at the grave site to destroy invisible vampires that would roam around. Other vampire’s hunters used silver holly bullets to destroy the vampires they chased

There are other tales related to fairies, werewolves and witches that are known be repelled by silver. In England and Scotland it was once believed that witches could take the form of animals, especially hares, and that it was invulnerable to all material shot from guns except silver. And one prescription calls for cutting a silver coin into four pieces before inserting it into the barrel of the gun. Excerpts from original accounts of this belief are given in The Werewolf by Montague Summers.