I wonder how much people tried to develop ballistae projectiles, perhaps once they had bolts/darts they were good enough and longer range just wasn't necessary?
In most cases if your goal is just to get some explosives over a tall wall, a trebuchet or siege tower is probably sufficient. On the other hand, if you have a lot of gunpowder, a cannon is also pretty fine on its own without any need to go up-and-over.
