Officers fired off dozens of rounds at three different locations around Dellwood, Jennings, Ferguson, and Florissant.
St. Louis County Police Chief Jon Belmar said the rounds were shot into a ballistic trap that was placed in front of a tactical armored vehicle.
The ShotSpotter system uses microphones placed across the area and can pinpoint the location where gunshots are fired within a four mile radius.
People in the area say they hear gunshots often but don’t call them in because they don’t know where they are coming from.
Belmar said the system will go online next week.