It's an artillery projectile, but I don't know anything more about it, except it would date later than the Civil War. Did you have a WWI or WWII training camp near where this was found? For a positive ID you need some exact measurements. I'm really guessing, but I think it looks like it's about a 55mm, just from remembering about how large a 40mm was 60 years ago when I was in the navy. The measurement needs to be the diameter in either millimeters or inches. Welcome to T Net.