Primer
The shock-sensitive cup pressed into the base of a cartridge. The firing pin strikes it and its flash ignites the powder. Sized small or large.
Primers come in small or large, rifle or pistol, standard or magnum. Use the size the cartridge requires and the brand your load was developed with. Primer brand changes pressure and velocity slightly.
Magnum primers run hotter and matter most with ball powders, large cases, or cold weather. Standard primers ignite stick powders fine for most precision work.