Witch-killing bullets are a type of hollow-point tipped bullets that have been injected with "witch-killing brew." The ingredients of the "brew" are not known, other than it requires fresh chicken feet. Unlike the normal spell, the bullets don't cause the witch to disintegrate, but have the same effect as regular bullets, killing witches who would be otherwise invulnerable to a bullet or other regular weapon. The bullet casings are engraved with a capital W to identify them.
In order to ensure Rowena breaks her attack dog spell on Castiel, Sam arms himself with witch-killing bullets, reminding Rowena that his gun is loaded with them when she hesitates. After Rowena breaks the spell, she disarms Sam of his gun and escapes.
When they go after the evil witch Sonja, Sam removes the small revolver from the Impala trunk he keeps loaded with witch-killing bullets. He later identifies the witch-killing bullets to Dean and the two decide they need to come up with a better name. With the help of a distraction from Melissa Harper, Sam is able to kill Sonja with two shots and then kill the qareen, breaking Sonja's curse.
The Winchesters are shown to have witch-killing bullets in the Impala trunk in a case with their silver bullets. Suspecting that Beth Roberts is a witch killing people, Dean loads his gun with the bullets to stop Beth. However, he learns that Beth is innocent and to Sam's shock, Beth later asks Dean out despite him nearly shooting her.
After discovering that Gideon Loughlin is a witch, Dean shoots him with a witch-killing bullet, mortally wounding him. Dean chases Gideon through the woods where he avoids a second shot and casts a memory curse on Dean before dying of his wound. The Winchesters later find the shell casing from the witch-killing bullet which Sam is able to use to identify it and the dead Gideon.
While going after Catriona and Boyd Loughlin, Sam arms himself with witch-killing bullets to force them to tell him how to break the curse, but he is defeated and disarmed. After waking up in the Impala, a memory-less Dean finds the witch-killing bullets labeled with a sticky note, as well as his gun. As Catriona prepares to kill Rowena, Dean threatens her at gunpoint and shows Catriona the note with "witch killing bullets" written on it when she scoffs at the idea of him using a gun against her. Dean then goes on to kill Catriona with a shot to the head and Boyd with one to the chest.
After learning that Tasha Banes is hunting a "borrower" witch, Dean shows Max Banes the box of witch-killing bullets in the Impala's trunk and loads his gun with them. Dean later kills the evil witch with a witch-killing bullet to the chest while she's distracted with Max.
Later, both Winchesters arm themselves with witch-killing bullets to fight the Plum sisters but lose their guns in the struggle. During the fight, Rowena kills the zombie Mrs. Plum with a witch-killing bullet to the head. However, this is merely coincidental as Rowena grabbed Dean's gun to shoot her in the head on Dean's order and the gun was loaded with witch-killing bullets at the time. Though Rowena has the chance to use the witch-killing bullets in Dean's gun to take out the Plum sisters, she instead chooses to set them upon each other with the Attack Dog Spell.
Sam and Dean bring along witch-killing bullets for their meet with Rowena. When Rowena is revealed to be an astral projection, Sam and Dean give chase to the real Rowena, and her bodyguard Bernard. Sam is able to slip past Bernard and follow Rowena outside to an alley where he holds Rowena at gun point, asking her to stop. Rowena tells Sam he will have to shoot her. Sam fires his revolver, but before the bullet hits Rowena, she freezes it in the air, and lets the bullet fall to the ground as she bewitches Sam to sleep.
- The witch-killing bullets were initially created to threaten Rowena. Ironically, in 12.11 Regarding Dean, her life is saved with one.
- When Sam uses the witch-killing bullets, he uses the small revolver he initially loaded with them which he appears to keep in the trunk ready for use. In contrast, Dean simply loads his regular gun with the bullets.
- While made with the potion from Bobby Singer's witch-killing spell, the bullets don't require the incantation (though the incantation may have been spoken while creating the brew), nor do they cause the witch to disintegrate like the spell version does.