Do you like it? I had my R&D people melt down one of your angel blades, cast it into bullets. Seems to do the trick.
Angel-killing bullets are bullets forged from melted-down angel blades. The bullets have the same effects as standard angel blades, and are able to wound or kill angels. They also appear to be capable of killing other creatures that are vulnerable to a standard angel blade.
Crowley appears before Naomi, shooting Ion's shoulder and killing Esper with a gun. He reveals to Naomi that he had his "R&D" people melt down an angel blade and cast the metal into bullets. He threatens and shoots at Naomi, who teleports away at the last second. With Castiel now in his custody, Crowley shoots him in the gut and has the angel Ion take Castiel to his office where he finds the angel tablet hidden inside Castiel's vessel.
After being left alone with Ion, Castiel digs out the bullet from his abdomen, and attacks Ion, driving the bullet slug through his eye, killing him.
When Castiel passes through a rift and arrives in an alternate reality, he is attacked by a horned tempter demon, but the demon is shot and killed by a nearby insurgent. Upon meeting the Apocalypse World version of Bobby Singer, Dean is excited to learn that Bobby's machine gun "Rufus" uses bullets cast from discarded angel blades.
Later when Sam, Dean, Castiel, and Crowley enact their plan to trap Lucifer in Apocalypse World, Dean borrows "Rufus". Pointing it at Lucifer, he tells him he has a bet on whether it will work on an archangel and begins unloading on Lucifer. The bullets prove ineffective as Lucifer is able to easily brush off the attack.
After being ambushed by a bounty hunter collecting humans for the angels, Dean and Ketch overpower the man and take his gear. Ketch remarks on the cutting-edge weaponry, and does not even recognize the angel-killing bullets.
Now armed with guns and angel-killing bullets, Dean and Ketch attack a POW camp where the Apocalypse World version of Charlie Bradbury is being prepared for execution. As the executioner is about to deliver his killing blow, Dean tosses a grenade before he and Ketch rush the entrance, shooting and killing all the angels in their path.
In Sam's nightmare, Dean kills several demons by shooting them with what are presumably angel-killing bullets.