Quote from SirLollington
I actually prefer the idea of having the detector consume an item every so many operations. Perhaps it should have its own inventory (3 slots or something) that it will drain over time as operations are done; and it would be possible to feed it using a hopper or a dropper, which removes the need of manually resetting the detector without removing the cost for running it.
Also, I think it should not be possible for the detector to see entities through non-transparent blocks. There was no mention of it, so I am bringing it up now.
Thanks for mentioning that. It was my intention all along but I forgot to clarify that.
Keep in mind that there is a "cooldown" to damage; two arrows hitting at the same time will cause only one to cause damage and the rest to bounce off without having an effect. The whole wall you described would probably not do much damage as a result; even if the travel time from the arrows on the top level is long enough to hit after this cooldown expired, that would be a total of ~6 HP damage, or 3 hearts. It's not really that overkill, and many, many arrows would go to waste since only two arrows actually do any damage if the target gets unlucky.
I don't know if you noticed but my most recent update to the OP included a change in the mechanics of the operation triggering. It now triggers one device at a time, cycling through clockwise beginning at the top, so having four devices connected serves only to expand the ammunition capacity (or create some kind of Russian roulette).
I support the idea, with some refining.
Thanks for the support!