If you live in Washington State, I doubt it is a cannonball. More likely a steel ball from a Ball Mill. It is a big rotating trommel with a bunch of those inside used to crush ore from a mine.
I would have to agree with Gollum. He's been very good on various forum ID's on various objects for a long while now. And, with all due respect for any forts there. Even if CW era in WA, yet there were no battles in WA of that war, eh ?. And hard to imagine they'd need cannon balls in indian wars of our west coast indian battles.
There has been a few true blue cannon-balls (even as big as bowling balls!) found at some maritime presidio places on the west coast (where they had batteries of artillery). But that is an exception, as those were lone military outposts of exploration times on the west coast.
Gollum probably got it right. In the late 60's I found two of those in SW Oklahoma. Lots of pre-Civil War military activity in the area so I was sure they were cannonballs. (I lost them during a move) Also lots of mining activity in that area. I always regretted losing them and then one day I stopped at a rock shop down by Canon City. They had 'Cannon Balls' for sale and explained they were actually balls from a crusher mill. And just dead exactly like what I had found years before. And after a lot of research about that particular area, I found that not one single group that used that Oklahoma site for 'Indian Fightin' had a cannon with them.
Allison asked me to examine the ball and give my opinion as to whether or not it is a cannonball.
Unless a ball has a recognizable artillery fuze in it, photos cannot tell me whether it's a cannonball or not.
In the photos, the seam on it "looks right" for a cannonball. BUT, the ball's precisely-measured weight and diameter are far more important. If the 1861 US (and CSA) Ordnance Manual's historical data records of cannonball sizes and weights (called the "Shot Tables") does not have an exact match-up for a ball, it's not a cannonball. I co-wrote an educational article with detailed instructions and photos, telling how to very-precisely measure the ball. You can view it online for free at: SolidShotEssentialsMod
When you've done the very-precise measuring and weighing, see if there's a match-up in the Shot Tables charts, at: www.civilwarartillery.com/shottables.htm
(I should mention, cw0909 already posted that link in this discussion-thead.)
Put simply... if a ball's diameter isn't with a couple of hundredths of an inch (.02-inch) of one of the cannonball sizes in the Shot Tables, it isn't a cannonball.
I should also mention, cannonballs have been found at places where a cannon was located but no combat ever happened there. Artillerymen have to do practice/training. Also, at more than just a few forts, cannonballs got thrown away when that kind of artillery became obsolete. In such cases, the ball's finder must do some research to provwe there once was a cannon at that location. And, the cannon's ammunition must be the same size ball that was found.