George, Probably a good site for this is:

Minié ball - Wikipedia

https://en.wikipedia.org/wiki/Minié_ball

The Minié ball, or Minni ball, is a type of muzzle-loading spin-stabilized rifle bullet named after its co-developer, Claude-Étienne Minié, inventor of the Minié rifle. It came to prominence in the Crimean War and American Civil War. ... The Minié ball was a conical bullet with three exterior grease-filled grooves and a conical

As stated the doctors had their own lingo and abbreviated the conical ball to con. ball. Because of some poor handwriting it does look like can. ball occasionally.