Thanks all--great help on this board. FYI--this is more from the firearm's expert once I emailed him the photo...
Man, seeing the picture is even more puzzling. I don't know why, but it sort of "looks" civil war to me -- though that is just my gut reaction with absolutely nothing to base it on.
That hole is deep, too -- not what I expected. It looks somewhat like a shotgun slug (which I believe someone mentioned), but it would be a whale of a gauge. And usually the "rifling" on those is more of a collection of slanted ridges along the circumference -- the barrel itself is not rifled in shotguns with the exception of a few modern guns designed just for shooting slugs.
Is the object lead? It almost looks like rust (or bronze) on it. If it were bronze or steel, then you might actually be looking at some sort of tool or part of a machine -- though what appears to be rifling argues against that.
The rim on the rear is interesting.... Is it possible you have HALF a projectile and the other half fit onto it? In such a case the hollow might have been filled with -- explosive? Yet for it to be apart argues against that as well.
I don't know -- totally perplexed. Let me know (if you would be so kind) if this mystery is solved to your satisfaction as I would certainly like to know what this is.