9mm is something like .355 caliber, so my swag would be you found a fired .38 caliber bullet. The actual size of a .38 bullet is actually smaller than .38, in truth it's .357/.358 which I suppose makes sense to some, but I scratch my head over it. The .38 measures the loaded brass, which isn't done on any other bullet I know about. The .38 has been around since the 1890's, and is still made today.