The system will have to distinguish real firearms from other powder-actuated hardware. Otherwise it's going to become useless anywhere near a construction site.
I think the best way to distinguish between firearms and nail guns would be with a man in the loop listening to the sounds (which they appear to have). Construction doesn't sound like a firefight.
What if you stab a person with a knife?
It's off-topic. It was designed and built to detect gunshots from real guns.