Bullet
The projectile, just the piece that leaves the barrel. Often misused to mean the whole cartridge; that's wrong.
The bullet sits at the front of the cartridge. After firing, it’s the only part that goes downrange. The case stays in the chamber, the primer is spent, the powder is gone as gas.
Bullets are described by weight (grains), diameter (caliber), and shape (boat tail, flat base, hollow point, spitzer, etc.). For long-range work, the shape and BC matter more than the weight in isolation.