Bullet Core
The inner mass of a bullet, usually lead, that supplies most of its weight; bonding it to the jacket improves weight retention on impact.
The bullet core is the dense interior, most often a lead alloy, that gives a projectile the bulk of its weight and therefore much of its momentum and sectional density. It is swaged inside the bullet jacket, and in a conventional cup and core design the two parts are held together only by mechanical fit.
How the core is joined to the jacket drives terminal performance. A bonded bullet chemically welds the core to the jacket so the two stay together through impact, which raises weight retention and keeps the mushroom intact. Some target and varmint designs instead use a harder or thinner core to tune how readily the bullet comes apart.