A muzzle loading firearm projectile is disclosed that is composed of a multi diameter, hollow base solid copper bullet, the rear cavity filled with a material of low-density, and a gas pressure seal that separates the bullet from the powder charge. The majority of the bullet shank has a diameter less than the bore diameter of the firearm barrel to allow for ease of loading and alignment of the barrel and bullet axis; a narrow ring of material larger than the barrel bore diameter but less than the groove diameter is located at the junction of the bullet shank and nose profile that centers the bullet in the barrel and positively positions the bullet over the powder charge regardless of orientation of the firearm. The low-density material filling the rear cavity of the bullet acts as an expansion medium when impacted by the rear gas seal during the firing process causing the hollow shank of the bullet to expand and lock into the barrel rifling.