need help identifying this strange object

[civilhiker]

Hi, today i went hiking around fort foote in MD. as we went down to the blockhouse i ran across a strange rock on the side of the old abandoned building overlooking the river. im not sure what this rock is, my best opinion is that it was used to grind down where the bullet goes in a brass shell?? it looks odd, but it strangely conforms to the human hand quite comfortably which led me to believe it was a civil war tool of some sort. here are some pictures, if anyone can help, please do.

 

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[mcb66]

Gorget preform. The native Americans would take a piece of cane and sand and start rolling the cane dipped in sand between their palms; kind of like starting a fire. eventually they would drill a hole in a hard stone like this.