Probably not 9mm. 9mm bullet is usually 0.355 diameter but can be 0.358. 38 special and 357 mag is also about 0.357. Just an observation, if it's a bullet it's what's called a hollow base bullet. that base looks to large for the round. the peening doesn't quite look like an impact, looks more like it's been pounded on. There are no rifling striations either. They should be parallel to the axis of the bullet. It may be a bullet but I'm not so sure.