Three types of projectiles known as Williams Patent "cleaner" bullets were used by the Union Army during the American Civil War in the standard .58 caliber rifle muskets. There was a fourth developed for use in the Union Repeating or "Coffee-Mill" gun. The inventor was Elijah D. Williams of Philadelphia, Penn, who filed an application for his patent on May 30, 1861. It was issued almost a year later on May 13, 1862 but field trials on his "improved" bullet had already begun.
The concept of the design was that the discharge of the musket would drive the concave disks forward thus expanding against the walls of the interior barrel removing excess black powder build up and soft lead residue from the lands in a rifled musket barrel.