As Oyeboten said, perhaps some sort of 1" Gatling gun round??
Here is a link to one with a lead projectile and inside-primed centerfire case.
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=197205285
Whatever it is, I would treat it as a HE projectile if you can see a nose cap joint.
There could also be a base detonating fuse / filler plug on the bottom of it you can't see without pulling the projectile out of the case.
Whatever it is, you need to decide whether or not it is worth the risk of having it laying around the house. Might be a good candidate for a one-way trip to a deep lake or river near you!
PS: The other curious thing I noticed is, for a steel projectile, it doesn't have a bronze driving band to engage the rifling in a rifled barrel.
waidmann in post #14 suggested it might be an 8ga "clinker" round used in foundry's to knock slag of the insides of blast furnaces. That is another possibility, as it apparently was designed to fire in a smooth-bore barrel, and Gatling guns had rifled barrels.
If it is a "clinker" round it would not have an HE projictile.
rc