Wow - That's a stunner.
I think I've got it. A 24 pound howitzer McIntyre repeating shell would be that size in the inner section.
This was a very unusual double or triple birsting shell that usually only appears in mortar shells.
The outer shell was a hollow iron sphere that has a time fuse and powder. Firing the cannon would light that fuse - and explode the outer shell - and lighting the inner fuse on this studded inner ball. after the number of set seconds went by, the second shell exploded. Thus the rain of iron would be lengthened by multiple bursts as the shell arched over it's target.
They were used in linited numbers at the end of the war around Richmond and gave by all accounts good service - at least in bursting multiple times.
Super rare and super cool - See page 263 in Artillery and Ammunition of the Civil War by Ripley - 4th edition