Lots of them turn up and some sell them on ebay for $10 each. But they are common.
Here is some history about those guns.
http://www.nps.gov/archive/anti/rifles.htm
Something else it could have been is grape shot for cannons. They would load grape shot and fire their cannons at the enemy and it was devastating. Kind of like a giant shotgun effect.
I did not take into account the weight, duh! Lol and the others are also right about it not being oxidized.
The North and south soldiers’ muskets did not shoot balls that big. (I stand corrected)
However there were civilians that did have guns that shot huge projectiles. Many were brought here from Europe, such as wall guns or rampart guns that did shoot balls over .80 cal.
There were Spanish muskets, that weighed at least eighteen pounds and generally had bore diameters of from 70 to 85 caliber, with a few specimens larger than 90 caliber.
There were many over .69 cal
The most famous British Army Musket - The Brown Bess, had a .75 caliber bore.
The French had wall guns in the 1800’s that shot .85 cal balls.
Also the early Americans shot large big bore shot guns up to 4 bore, with patched balls.
http://www.gunweek.com/2001/feature0201.html
There were Remington rolling block rifles in .81 caliber that the French used also.
However despite that; if its steel and not lead then its not from one of those guns and is likely as CIH stated a ball bearing from farm equipment.
But I did find a .82 cal musket;
http://www.proxibid.com/asp/catalog.asp?aid=6701
(Bottom of page)
“British Colonial Maritime Provinces Flintlock Musket Marked: Thomas Lynch
.82 caliber, 56 1/4-inch barrel with three barrel bands and British proofs near breech. Unmarked lock plate.”
Here are some links about large muskets;
http://209.85.173.104/search?q=cache:e8HcWQybLQMJ:www.geocities.com/milsurpunderground/French.html+An+81+caliber+musket+ball&hl=en&ct=clnk&cd=1&gl=us
http://www.angelfire.com/ga4/guilmartin.com/Weapons.html