Prior to the Civil War, most of the military was armed with smoothbore muskets that fired round shot made out of lead. This is one of the reasons that accuracy was generally a wishful thought and while getting shot but a ball was not pleasant it was nothing compared to what was coming.
During the Crimean War, 1853-6 (a war we may look at later, but not now) a new shot was being used to great effect. This type of shot was named the Minié ball, after the doctor. In the picture above you can see what the Minié ball looked like, and also a piece of round shot.
The Minié ball was conical with rings around the base and a depression in the bottom, it normally would be slightly smaller than the barrel of the musket to make loading easier. Once fired the soft lead would expand and fill the gap around the bullet making its own sort of wading. When this new kind of ammunition was introduced on both sides during the Civil War, something else became very evident.
The round shot was easily deflected once it hit the target. It would tend to bounce around inside a human body before becoming lodged somewhere. The Minié ball, which was heavier and traveled faster, would simply cut through the body like a ripe melon. If by chance it struck a bone, the bone would simply shatter. Leaving amputation the only way to save the person .
Eventually, the Minié ball would be replaced by something even more deadly. For a time though it reigned and there was nothing mini about it.