This one looks like it has four distinct rings, and the cavity is just as you see it .. what you are seeing in the cavity is lead, not dirt, with a small hole in the center. Could be a casting flaw. Any thoughts? Thanks!
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No offense intended, but in bullet-ID terminology, those aren't rings, they are grooves. (A "ring" on a bullet's body is a raised ridge or band.)
In my opinion, the lower one appears to be a "crimping" groove, where the lip of a metallic cartridge gripped the bullet. So, despite the unusual cavity in its base, I believe you've got a metallic-cartridge bullet.
In the photos, your Caliper shows the bullet's diamter as being .657 -- an extremely unusual size. It doesn't match up with any known caliber of bullet. But your other photos seem to show some irregular-shaped "swelling" above the bullet's base. Please re-measure your bullet's diameter just slightly above the swelling.
Then, post your photos & info in the Bullets category at the following link. The Civil War Projectiles forum at bulletandshell.com is the best place on the internet for identification of civil war era (and "slightly" post-civil-war) bullets. Civil War Projectiles Forum - Index
When I first read this discussion, I immediately thought of a Snider-Enfield, because it's one of the very few "large caliber" 4-groove bullets. But the Snider is .577 caliber, and Creskol's bullet's diameter is significantly bigger than that.