I would love for this to be optimized to the point that its portable and can be carried around, assembled on the side of the mountain, and used as a low-cost, efficient way to throw things from one mountain to another, maybe for avalanche control, or cloud seeding, or indeed for seed bombs intended for re-wilding hard to access places, or something.
It just seems like such an obvious tool, rather than a weapon.
You'd probably be better off using a rail gun if you want a carriable launcher
if you want low cost and portable, chemistry (i.e. gunpowder) always wins over gravity powered.