I can tell you it's definitely not a musket ball due to it being out of round and the huge casting seam, plus it's HUGE... I wonder if it may be case shot, but I'm far from an expert on those. You may want to PM CannonballGuy and ask him to have a look
The finder (Fernald1) now says he thinks the ball is not made of lead, saying "It's green under the filth," which suggests it is copper or perhaps brass. So before speculating any further about its ID we need to determine whether it is made of lead or brass or copper, etc. The photo showing the ball with a ruler indicates the ball is very close to 1.0-inch in diameter. A 1-inch lead ball weighs 1497 grains, which translates to 97 grams, or 3.42 ounces. Fernald1, please weigh that ball on a Postal Shipping scale or Jeweler's Scale, and tell us its exact weight.
If it does turn out to be a 1.0" lead ball... as NOLA_Ken said, it is too out-of-round and heavy-seamed to be a musketball. As he guessed, it THEORETICALLY could be a civil war US Navy artillery heavy-caliber Case-Shot ball or Navy Canister-ammo ball. But those are extremely unlikely to be found in Central Massachusetts. The only possibility for it being an artillery ball is, it might be a Revolutionary War era or War-Of-1812 "Quilted Grapeshot" ball.
Or, as A2coins suggested, it could be a fishing-net weight. Those tend to have a deep narrow knife-slit cut in them, into which a line is pressed, and then the slit would be pressed tightly closed to hold the line. Need to look super-closely for any sign of the closed slit.
Copper is around 25% lighter than lead, and brass is about 30% lighter than lead.