Abstract: A projectile or bullet for a muzzle loading firearm that has a fixedly attached skirt as a gas seal. The projectile is a full-bore bullet that has high velocity and high accuracy and is easy to load. The bullet includes a rear body portion covered with skirt made of a different material than the body to form a gas seal when the projectile is fired. In a preferred embodiment, the skirt material is plastic, paper or nylon. The body portion of the projectile may be covered by a jacket. The nose portion of the bullet may incorporate a ballistic tip. In some embodiments, the rear of the bullet body is indented to allow the expanding gases at ignition to expand the rear of the bullet to further seal the combustion behind the bullet and to further seal the skirt to the bullet.