The Collaborative International Dictionary of English v.0.48:

Minie ball \Min"ie ball`\ [From the inventor, Captain Mini['e],
   of France.]
   A conical rifle bullet, with a cavity in its base plugged
   with a piece of iron, which, by the explosion of the charge,
   is driven farther in, expanding the sides to fit closely the
   grooves of the barrel.
   [1913 Webster]