Brass
Also: Cartridge case
The metal case that holds powder, primer, and bullet together. Brass is reusable; aluminum and steel cases are typically one-shot.
Brass is the foundation of repeatable handloads. The case dimensions, thickness, and lot consistency determine how reliably the rifle chambers, how cleanly it ejects, and how tightly velocities cluster.
Premium brands (Lapua, Norma, Peterson, ADG, Alpha) sort to tight tolerances and last 5-10+ firings if you anneal between cycles. Bargain brass might be perfectly serviceable for 200-yard work but produces inconsistent ES at distance.