Back in the early 80's there was a local cannon club. They would bring their guns to my hay field for shooting
matches. There were guns all the way from full a sized 12 pounder down to one inch bore. The full sized Napoleon
didn't have a 4" bore, they were shooting lead balls around 2 inches if I remember correctly. There were a couple
of guns that were made from 40 MM gun barrels, and they fired breech loaded bullets that look just like your find,
hollow base giant minnie balls so to speak, including rifling marks like on your find. The barrels came off scrapped
Navy ships. The yard scrapping the ship had to blow holes in the gun barrels every 5 feet, so there were a number
of guns made from Navy gun tubes. One of the guys cast his bullets out of a more brittle lead, I don't know what,
but for sure scrap, perhaps battery lead, anyhow, when he fired the skirt on the bullet would split and the darn thing
would whistle real loud, which was fun. Anyhow, your find could be somebody shooting their home made cannon, which
would make it a rare find, unless you were on my old place. I've seen bullets that look exactly like yours.
