In other words, 9mm

The entire mass of Earth, compressed down to the size of a bullet. That's actually a very vivid illustration, unlike most of the other large numbers that get thrown around in astronomy.

Its actually compressed down to a point. A singularity. The 9mm is just the radius from the point from which nothing can return.

For all intents and purposes of astronomic calculations, you can pretend it's 9mm in size, because A) the difference between a 9mm sized object that behaves like a black hole and a singularity with a 9mm event horizon is minimal and B) it's a 9mm black hole going 50km/s+, most math will result in the extinction of human kind anyway.

To be fair, we don't know what is behind those 9mm. Einstein's equations predict a singularity but afaik there is consensus that those don't hold there.

A rather lethal 9mm bullet. Would it just punch right through the earth if it hit or would it stop and then eat the earth from the inside?

it would just spagettify the earth long before it reaches the center.

Would it? If it were one earth radius away from the surface it would exert as much pull on someone on the surface as the earth would, right? Things couldn't start becoming spaghetti before that point, and that's pretty close to earth; surely it'd reach the earth's center very shortly after that.