Looks like a slug out of a shotgun but I am sure its not, I'm sure there are someone on here that can tell ya exactly what you have. Good Luck Wish I Could Have Helped Ya More. Good Luck
In the lefthand photo, at the two o'clock position, there appears to be a little protrusion on the rim. If that is the case, it may be a seal of some sort. I remember as a kid, rolls of wire use to have their manufacturing tags affixed with lead blobs like that.
Looks like a round ball that somebody flattened out to re-purpose it. What's the diameter?? Keep in mind that round balls were used in pistols, hunting long arms and shotguns as well as muskets long before and after the Rev war.