That's it. Projectiles (mostly) don't have a trajectory from the simulation point of view, they are just distant delayed hit. Then a trajectory is rendered. The choice was a parabolic trajectory. <Gravity> is the graphical parameter which will adjust the height of the parabola. For now, artists have to tweak that so it fits what they want to see. Don't bother with anything else. More have to be said, but not enough place on that margin.