primer
Ammunition component, legitimate firearms/manufacturing term
The small circular piece of metal (usu. brass) at the base of a bullet, where the firing pin in the chamber of a firearm hits to make a spark and ignite the bullet's powder.
Student: Why can't we use that bullet? It's a 9x19mm, isn't it? Instructor: Yes, but it's a dud. Student: What? Instructor: As you can see, there is a dent in the primer. It was hit by the hammer, but did not spark, and therefore did not eject from the barrel.