I co-wrote a very-detailed Educational article at:
SolidShotEssentialsMod
which gives instructions for being able to determine with certainty whether a metal ball is an Artillery ball (cannon ball, Grapeshot-ammo ball, Canister-ammo ball, or Case-Shot ammo ball) or is a Civilian-usage ball (Mining Industry ore-crusher ball, Sports Shot Put ball, Ornamental-Ironwork ball, a ball-bearing, etc).
First, you'll need to clean enough of the rust-encrustation off of a dug ball to be able to ACCURATELY measure its original diameter, in hundredths-of-an-inch. Then, please read and follow the instructions in the article I linked to in the paragraph above.
For measauring ball which are less than 3.25-inches in diameter, an inexpensive Digital Caliper will do the job. For balls bigger than 3.25-inches, you'll need a Diameter-Tape (which is also known as a Pi-Tape).
Meanwhile... the answer to your question ("Is this grapeshot?") is no - because in your photo it appears to be smaller than 2.02-inches in diameter, and thus it is too small to be a Grapeshot ball. Check the size-charts for Grapeshot balls at
www.civilwarartillery.com/shottables.htm
