The Minie ball is a type of muzzle-loading rifle ordnance. It was originally designed by Captain Claude-Etienne Minie of France in 1849. Made from soft lead, it was small enough to fall downwards, past the rifling and any detritus from the prior shots. It was then rammed home with the ramrod, which ensured that the hollow base was filled with powder, and the rifle or pistol was ready. (Leaving an air gap was a good way to destroy the gun)

When fired, the expanding gas pushed hard on the soft base, pushing it both out of the gun, and outwards to the rifling. This increased the diameter of the bullet such that it engaged the rifling that it had missed on the way down, causing it to spin, which greatly increased accuracy. The better seal formed also meant that the velocity was more consistent, as well as removing some of the previous crud, hence slowing the build-up.

These advantages ensured that the bullet was rapidly adopted. In was more powerful (less leaking gas was wasted), more accurate (it engaged the rifling well), and more lethal (since it was longer, and not limited to the mass of a simple sphere, it could be heavier) yet cost no more and added no extra steps to the loading process.

They were widely used during the American Civil War and first came to prominence in the Crimean War of the 1850s.