If it is what I think it is you have a rare and very cool find. I believe you have the fired base of a Gardiner Exploding Musket Shell, if you search that term and select images you'll see what I mean. An interesting note - there is probably vert fine raised lettering cast in along the edge of the base reading "S. Gardiner Jr. Shell Patent" or "S. Gardiner Jr. Shell Patent Secured" (I've seen references to both. These were used by the Union and only issued (as far as I can tell) on a very limited basis - with Chancellorsville and Gettysburg being the most notable. They were then withdrawn because they produced horrible wounds and were deemed "too fiendish" for use - and this from folks who saw wounds from all sorts of musket balls. Seriously - G@@gle Gardiner Exploding Musket Sh
ell, there are some seriously fascinating forum threads and articles on this. From the size I'd guess its the 58 cal example. If you wanted to rule it out take the base ring diameter, if it is way over 0.58 inches (it might have expanded some due to being fired, exploding, and developing a corrosion layer) then I'm wrong. The best way to confirm it would be to look for the lettering.