Wad slugs[ edit ] From the left, plumbata discarding sabot 1 ; plumbata slugs 2, 5 ; wad slug 3 , sabot slugs 3, 4 A modern variant between the Foster slug and the sabot slug is the wad slug. This is a type of shotgun slug designed to be fired through a smoothbore shotgun barrel. Like the traditional Foster slug, a deep hollow is located in the rear of this slug, which serves to retain the center of mass near the front tip of the slug much like the Foster slug. However, unlike the Foster slug, a wad slug additionally has a key or web wall molded across the deep hollow, spanning the hollow, which serves to increase the structural integrity of the slug while also reducing the amount of expansion of the slug when fired, reducing the stress on the shot wad in which it rides down a barrel. Also, unlike Foster slugs that have thin fins on the outside of the slug, much like those on the Brenneke, the wad slug is shaped with an ogive or bullet shape, with a smooth outer surface.
Unfortunately, the accuracy of a lead ball fired from a smooth bore barrel is pretty sad. Hitting the target is problematical and precise bullet placement is nearly impossible except at very close range. Also, a lead ball has a very poor sectional density SD , and consequently poor penetration. There had to be a better way. The answer, of course, was the rifled barrel. Imparting spin to a projectile to stabilize its flight was a quantum improvement in accuracy.