[Intermernet]

> the ballistic will eventually come back to earth (unless it gets captured by another body's gravitational field)

Although ridiculously difficult and inefficient, it's possible to achieve orbit with just a gun, and another body's gravitational field. It doesn't need to be captured by said field, it can just be influenced enough to stabilise it's orbit.

> In a three-body or larger system, a gravity assist trajectory might be available such that a carefully aimed escape velocity projectile would have its trajectory modified by the gravitational fields of other bodies in the system such that the projectile would eventually return to orbit the initial planet using only the launch delta-v [1]

[1]: https://en.wikipedia.org/wiki/Space_gun#Getting_to_orbit