Calling in Artillery Strikes

After completing the Minutemen quest Old Guns, you'll be able to call in artillery strikes from nearby settlements with an artillery piece built. In addition, you have to assign a settler to the piece to make it functional. Artillery pieces add 6 defense points to your settlement's overall defense rating when occupied.

Artillery pieces have a range of five grids in the map so you'll have to build several of them across different settlements to ensure you'll have the best coverage possible when you need Fire Support.

To call in an artillery strike, you have to mark the location using an artillery signal grenade. Sanctuary seems to produce this regularly and you can collect 10 of these grenades from the workbench after some time has passed. Take note that the artillery strike may have several shells landing in different landing point and intervals so avoid using it when you're too close. Also, there's a considerable delay when the signal grenade has been thrown and the actual landing of the shells so it's quite useful for siege-ing enemy positions.

Fire for effect!