We have CCTV networks installed and maintained and they are left to run unmonitored until evidence is required.
Further to that the police did a great job of maintaining the GATSO speed-camera network with film until it was updated to digital - they didn't seem to have an issue shooting with and reloading those cameras. Potentially we could have an Auto-Turret in every classroom, and Auto-Turret installed over the desk of every convenience store.
As for triggering enough, what is the current American gun deaths and injuries rate per year? 30,000? I expect that number to half in the first year. Second year down another 25% and third year I would expect it to be down to just 15% of the original figure. My main reasoning for gun crime not to be eradicated is that the parts of gun crime that are eradicated will be where the Auto-Turrets are. Populated, built up areas where people firing guns in public is a no-no. Gun crime would be pushed out in to unpopulated areas where there is a lower chance of innocent bystanders being caught up in things like shooting sprees.
In Gun We Trust.
Auto-Turret doesn't have to fire right away, it can warn, even release warning shots a few inches from the perpetrator to attempt to convince them to stop:
View: https://youtu.be/blzmPZ7ZNJc?t=15s