Gilding Metal
The copper-zinc alloy most bullet jackets are made from, hard enough to take the rifling cleanly without heavy bore fouling.
Gilding metal is a brass alloy, typically around ninety-five percent copper and five percent zinc, that forms the bullet jacket on the great majority of jacketed projectiles. Its hardness sits in a useful middle ground: stiff enough to engrave on the rifling and hold its shape down the bore, yet soft enough to seal against the lands and grip without tearing.
That balance is also why gilding metal is the standard rather than pure copper, which is softer and tends to smear. Even so, every jacketed bullet leaves some trace behind, and managing copper fouling from gilding metal is a routine part of keeping a precision barrel shooting to its potential.