Projectile
The piece that flies: a bullet, a slug, a pellet, a sabot. "Projectile" is the umbrella; "bullet" is the specific kind for rifled barrels.
Use “projectile” when the reader needs the generic term. Most ballistic discussions for rifles use “bullet” because that’s what a rifled bore fires; “slug” usually means a smoothbore (shotgun) projectile; “pellet” is air-gun or shotgun shot.
In ammunition labeling and reloading manuals, “projectile weight” is shorthand for bullet weight, measured in grains.