What's the cheapest device one could build that isn't a firearm, but could reproduce the sound of a gunshot with enough fidelity to fool this thing? Bonus if it can do a nice AK-47 machine gun sound or something like that.

Assume that the device requires electricity. It probably should be wifi-connected, so that if you can get it close enough to a Starbucks or something, you can control it remotely. Should ideally be small and unobtrusive so that it's not easily spotted. Shouldn't need any refillable fuels.

Would be fun to put the plans up on the net so that many people would build and field these.