Is this a cannonball?

[Enohcs]

Kids dig it up in yard (belmont, MA). Weighs 2lb 2.5oz, and is about 2.4 inches in diameter

 

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[Icewing]

Until Cannonballguy gets here you can start comparing yours to the measurements in these charts.

Cannon bore, shot, and shell diameters for smoothbore guns

 

[TheCannonballGuy]

I co-wrote an educational article just for folks like you, the finders of a rusty iron ball which might-or-might-not be a cannonball or other Artillery ball (Grapeshot ball, or Canister-ammo ball, etc).
SolidShotEssentialsMod
It has helpful photos and detailed instructions... plus a link to the civil war artillery Shot Tables data charts, which tell you the very-exact (in hundredths-of-an-inch) diameters and weights (in tenths-of-a-pound) for the actual cannonballs, Grapeshot, and Canister-ammo balls used in America from the Revolutionary War through the civil war.

I've given you the information above just in case you're still interested after I tell your ball is definitely not any type of Artillery ball. I'm 100% certain about that diagnosis because the diameter and weight measurements you gave for the ball prove it is made of steel. Civil war (and earler) cannonballs, grapeshot balls & etc were made of simple cast iron, not steel. The US 1861 Ordnance Manual's Shot Tables at the link given above say that a solid (not hollow) cast-iron 18-Pounder caliber Grapeshot ball which is 2.4-inches in diameter (same as your ball) weighs 1.8 pounds (1 pound 13 ounces). But yours, at the exact same size, is 5 ounces heavier. Therefore, it cannot be made of cast-iron... it's got to be steel, which is a denser, heavier iron alloy than simple old cast-iron.

Being steel, it is most likely a large ball-bearing. Before giving up, you could try re-checking its weight on a different scale, to see if it comes down to 1 pound 13 ounces.