A relative was showing a collection of bullets, found at Farmville, Va, Battle of Cumberland Church. A few interesting ones, one that had bite marks and the one pictured that I have not seen before.
While it has come to be known as a "Cleaner" bullet, that was not its original use. The harder zinc alloy disc was pushed forward into the softer lead bullet when fired, expanding the softer lead to create a tighter seal between the bullet and the barrel. That gave it greater trajectory, velocity and precision.
Williams patented 3 different versions of this bullet.