The hollow base of the cone-shaped minie ball (named for French inventor Claude Minié) expanded when the gunpowder ignited, thereby catching its grooves in the interior rifling of the gun and increasing the velocity and accuracy of the bullet. The longer, effective firing range of minie balls also turned mass infantry assaults into mass slaughter until military tactics caught up with the destructive power of the new technology. The ubiquitous minie balls have been collected as battlefield souvenirs ever since.
Information from Library of Congress
Private J. Luman’s Skull…
"Wounded at the battle of Mine Run, Virginia, on November 27th, 1863, when a minie ball passed through his skull. He was treated in the field hospital for several days before being evacuated to the 3rd division hospital in Alexandria. By December 8th, Private Luman was comatose and Surgeon E. Bentley applied a trephine and removed the splinters of bone associated with the wound. His condition failed to improve and he died five days later."
-The National Museum Of Health And Medicine