I bought this musket ball at an antique store in Gettysburg,PA, about 4 years ago. They said it was found after the Battle of Gettysburg. I forget what I paid for it, and it is approx. 15 mm across.
There were approximately 80,000 shots fired at Gettysburg. That little store has sold over two million of those balls that great-granddaddy picked up right after the fighting stopped!