Secondly I searched for ballistaes in the forum but couldn't find any (except a topic in design philosophies), so I'm guessing it's not fully mentioned as a suggestion
I would much rather have a more specific block/entity for firing the broadhead -arrows than the dispenser,
mainly because it sucks having a block that is limited to only 6 directions in space.
The dispenser is fine for traps inside complexes or maybe building a battleship just for fun, but there is a clear gap between manually shooting and self firing mechanism.
This gap in usability is mainly in allowed angles to aim and to acquire target, and the first one would do tons to cover this gap.
My suggestion is the design smaller ballista mounted on a wooden frame/stance (forgot the right word <.<;;)
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1. Needed to load ammo in a size restricted ammo slot to restrict independence of ballista
2. mechanical power needed between shots to rearm the ballista, supplied either by adjacent handcrank or by axles from below. (to restrict fire speed and size of selfiring contraptions)
3. A GUI (the thing that pops out when right-clicking chests, craftingbenches etc.) for ammo and aiming.
4. Aiming could be made simple by a control interface composed of a 'clock' interface to select the
'phi'-angle (NSWE-plane) and a slider for the 'theta'-angle(up-down angle), a good idea though is to limit increments of angle to 5, 10 or 15 degrees as this gives the impression of a 'cleaner' interface.
5. Should only be able to fire broadheads (and MAYBE arrows) this IS a specialized tool and shouldn't have more functionality than a dispenser or a BD, although as it is loaded by mechanical power it could maybe be more of a hardhitter...
6. I believe that being something that faces more than the 6 allowed faces/angles for cubes, it would probably work better as a entity, or maybe a comb. of both?
What are your thoughts?