Looks like a hollow base wadcutter (target bullet) to me. Diameter measurement and weight would help narrow it down. Just eyeballing it , I'd guess....and a WAG it's a ,38 caliber? Contemporary design most likely. Monty
Agree with Monty and Ironpatch...post civil war. Springfield brought out a "Trapdoor" in 1873 that utilized a bullet similar to yours but without diameter, length and weight....who knows?
That would be approximately .44 caliber. Could be .44 Magnum, .44 Special and/or a .44/40. Most likely a semi-wadcutter that had the nose smashed on impact. Monty