Bullet Jacket
The metal skin over a bullet's core that engages the rifling and governs how the projectile behaves in flight and on the target.
The bullet jacket is the outer metal shell, usually gilding metal, that surrounds the softer core and forms the bearing surface riding the rifling. It protects the lead from the heat and friction of the bore, lets the bullet take the rifling cleanly, and shapes the nose and base that determine flight.
Jacket thickness and uniformity have a large effect on both accuracy and terminal behavior. A thin jacket peels open readily for rapid expansion, while a thicker or tapered jacket holds the cup and core together for deeper penetration. Concentricity of the jacket wall is one of the quiet factors that separates a match bullet from a bulk one.