With the addition of static cannon mounts, cannons can now be built aiming directly downward. It would be nice if cannons that are pointing downward could use gravity to drop loaded shells below them without needing to accelerate them using propellant.

Not implementing mostly out of concerns for integration with ship mods and such - I don't want someone to have to worry about defining "down". IMO it's also a stretch, I would make dedicated bomb bay blocks if I wanted to add this feature (and I don't have any plans in the future to do so, at least for this mod).