Lead ball with seven flats

[Roff]

I found this lead ball at the site of a 140+ year old mining town (it existed in the late 1860's) in the mountains in Idaho. I'm wondering if anyone can tell me how it acquired the 7 flat sides. Several people have told me it has not been fired, as evidenced by the lack of damage or distortion. I am wondering if the circular groove was caused by a rammer on a black powder pistol.
I have another one that is virtually identical to this one, except that it is in slightly worse condition.

 

[Jack]

The circular groove might well be caused by a rammer on a revolver.
That would not explain the 7 flats, however. Revolver cylinders don't have the corresponding flats. Revolver barrels are where the rifling is.
A round ball like that would not necessarily distort upon firing, other than to be inscribed by the rifling in the barrel. Distortion would come when it hit something after being fired. A round ball ball fired into a a soft substance, like a mattress, might not distort at all.
Elmer Keith wrote about recovering undistorted bullets that had been fired into snowbanks, and recovered in the spring when the snow melted.

 

[tbuckley]

What are the measurements for the ball? If it is .54 caliber, it looks like it has been fired in a M1841 Mississippi Rifle. The Mississippi Rifle had 7 groove rifling.