Sometimes, to identify an unknown object, we must first eliminate some of the ID possibilities.
I mention the following possibility because you live in the Atlanta area, where civil war artillery activity happened. At first glance, because it is a piece of thick, curved lead "belt" with smooth flat sides, it resembles a broken-off piece of an exploded civil war artillery shell's lead band-sabot. But that cannot be what it is, because the "inner" side of its curved body is "plain," instead of having attachment-ridges or notches needed to attach a lead belt onto the projectile's iron body. Also, does not show any cannon rifling-marks on it, which WOULD be present on a lead band-sabot from a fired artillery projectile.
Another possibility is a broken-off piece of thick lead "pipe-joint seal" from antique cast-iron drainpipe plumbing. I mention that possibility because you said there were buildings on that that spot in the mid-through-late-1800s. Because I worked as a Plumber's helper for about a year, I've seen antique cast-iron drainpipe lead seals. That is what the object you found looks like to me... except, such pipe-seals do not have the knobby projection we see on the outer side of your find. So, I have to discard the second ID-possibility.
I'm sorry that I cannot tell you what the object is. All I can do is tell you two things that it isn't. I hope that helps at least a little bit.
