I have found a few of these also when hunting arrowheads. A few still have peck marks. I have found both made of granite and flint.
I could see the view that these could be cannon shot. Depending on the location found and dimensions of the ball, the view could go more towards one or the other.
Unlike the small marbles sometimes found, the larger flint is extremely smooth, Two, including the flint also have flat spots. One of the granites hardstone has three of varying size.
This is the polished flint
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Here is a very round harstone ball found a few winters back, you can still see some peck marks in the lower area, sort of shadowed.
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The hardstone ball was found by my house. From one angle it looks like a very round ball. Turn is and you see the flat spots. I have tried to show both angles in these three pictures. The flat spots are different sizes. On most sites where I have found these, I have also found pestles and axes. The pestles were both roller and hoof pestles. Where the flint ball was found I have found bell pestles. A few highly styles with well-defined bases.
The flat spot is in front, sort of towards the camera.
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The larger flat area showing on top.
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flat area on right is clear, another in the lower left.
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