Expansion
Also: Terminal Expansion · Mushrooming
The controlled mushrooming of a bullet on impact that increases its frontal area and transfers energy into the target.
Expansion is the way a bullet deforms on impact, peeling its nose back into a wider mushroom that boosts frontal area and slows the projectile inside the target. That growth is how a hunting bullet converts velocity into a larger wound channel and a faster transfer of energy, a core concern of terminal ballistics.
Bullet design controls when and how much a projectile opens. A hollow point uses an open cavity to start the mushroom, while a jacketed soft point exposes lead at the tip to the same end. The engineering challenge is reliable expansion across a range of impact velocities while still holding enough weight retention to penetrate.