Fullstock, you very helpfully provided three well-focused closeup views of it... a sideview showing the "axial" deep wide flatbottomed groove, a baseview, and a topview. But I need to see the other side of it. Until you post a pic showing its "other side" I cannot complete my analysis of it. For the moment, one thing I can say with certainty is that it's is not a Whitworth bullet. That rifle had a hexagonal bore, instead of rifling-grooves... which "seem" to be visible on your bullet.
Very interesting. It's so badly deformed, it almost doesn't look like a bullet, but two clues say it is. It has a base cavity, and (more importantly) it shows the effect of being either:
1- VERY hard-rammed
2- Attempted removal (but unsuccesful) by bullet-worming with a Double-Helix bulletworm, or
3- Fired with the ramrod left in the gunbarrel.
At the moment, I'm leaning toward explanation #2.
Need several more photos, please, each shot from a different angle.
