There are quite literally multi-millions of of iron and steel balls which were manufactured for civilian and industrial purposes. To correctly distinguish those "not-cannonballs" from actual artillery balls, we cannonball collectors have to rely on making super-precise measurements of a ball's weight, diameter, metal composition, and "perfect sphericality." We then look for a matchup in the historical cannonball size-&-weight data charts in the US Ordnance Manual of 1861.
I co-wrote a detailed article on how to distinguish actual "authentic" cannonballs from the many civian-usage lookalikes. You can read it online, for free, at:
SolidShotEssentialsMod
For the moment, I'll go with what you've told us thus far. The Ordnance Manual (whose data-charts tell us the very-precise diameter and weight of cannonballs, Grapeshot balls, and Canister-ammo balls used in America from the Revolutionary War through the civil war), the nearest cannonball weight to your ball's 6 pounds 4 ounce weight is a 6-Pounder caliber Solid-Shot. It weighed precisely 6 pound 1.6 ounces ...or an ounce or so less if there were casting-flaw airbubbles trapped inside the ball. There was no solid (not hollow) cannonball which weighed in the neighborhood of your ball (6 pounds 4 ounces).
To view the 1861 Ordnance Manual's "Shot Tables" very-precise diameter and weight charts, for free, go here:
Cannon bore, shot, and shell diameters for smoothbore guns
Also, a 6-Pounder caliber cannonball was 3.58-inches in diameter. Your ball is smaller than that, even with the rust-encrustation which increases its original metal diameter.
Sorry to have to disappoint, but based on the available evidence, I have to say your ball fails two of the crucial tests (diameter and weight) for being an authentic (actual) cannonball.
