heres some pics. i put a 30-06 round by it for size comparison.
Wonderful artifact! That's a shaft smoother. They were heated and utilized to smooth and straighten shafts - often used in pairs. The smaller lines are solely for decoration. Where did you find it? Although they do occur in Oklahoma, they are pretty rare here. These are commonly found out west, most examples made from jadeite or steatite. Yours looks an awful lot like steatite.