It's actually spelled " Minié ball "
It has much heavire mass than a roundball of the same caliber, and therefore a much more powerful penetration. It may not be as stable ballistically as a roundball
From Wikipedia:
The Minié ball (or minie ball) is a type of muzzle-loading rifle bullet named after co-developer, Claude-Étienne Minié. It came to prominence in the Crimean War and American Civil War.
Invented in the 1840s by the French Army captains Montgomery and Henri-Gustove Delvigne, it was designed to allow rapid muzzle loading of rifles, an innovation that brought about the widespread use of the rifle as a mass battlefield weapon. The design of the ball had been proposed in 1823 as the Cylindro-conoidal bullet, but was not adopted.
It was a conical-cylindrical soft lead bullet, slightly smaller than the intended firearm barrel's bore (see caliber), with (originally) four exterior grease-filled grooves and a conical hollow in its base. As designed by Minié, the bullet had a small iron plug in the base whose purpose was to drive forward the bullet and, under the pressure of powder gases, obturate the bullet to fill the hollow space and expand the lead skirting to grip the barrel's rifling. As finally adopted by the United States Government before the Civil War, however, the skirt of the bullet base was made slightly thinner and the plug was omitted, as the pressure of the powder gas alone was sufficient to expand the skirt to engage the rifling. Also, as adopted by the U.S. Government, only three exterior grease-filled grooves were used, instead of four.