My late father got these many years ago and said they were at one time on display in Duluth, Minnesota. Supposedly they were saved from getting melted down at the steel plant during WWII. I have tried to find where the cannon was located, but have never been able to do so. Maybe you will have better luck.
Over the years he gave them out to friends, so only 9 remain. Some are in rougher shape, so I imagine the ones at the bottom of the pile rusted more then those at the top. The one you see in the photo is the one you will get.
Ball is solid steel and weighs 19 pounds, 2 ounces. It is 5 inches across. I do not have a micrometer that size, so it is not an exact measurement. The paint and rust may throw off the size by a fraction as well. It's very close to 5 inches.