I think the detection and logging aspect has a lot of promise as a cheap and widely deployed system.

I'd worry about the liability from false positives with a active response system. Even something as limited as locking the doors to a room to contain the shooter could lead to liability from someone being trapped with the shooter.

If you assume a large indoor setting, something like a mall or school, maybe combine the detection network with a evacuation guidance system that leads people away from the incident. Mass shootings are pretty rare so maybe you could use it as a general "smart evacuation" system. OTOH there's probably a crowd psychology problem in there that makes that unfeasible.