Old Guns

Old Guns is a quest and achievement/trophy in Fallout 4.

Quick walkthrough
Quest Can be Received from Preston Garvey in Sanctuary at a certain point in the game. Alternatively, a few days after you secure The Castle, when you tune to Radio Freedom you will hear the radioman and Ronnie Shaw asking for your presence in The Castle, which will also begin the quest. After receiving the quest, head to The Castle and talk to Ronnie Shaw. She'll guide you to the old General's room where you need to use the Workshop to clear the rubble. Inside, you'll find a turret and Sarge. Sarge will boot upon seeing the player, giving a few seconds to land a couple of power shots to weaken him before the battle begins.

After defeating Sarge, open the door with the button on the wall and take the Artillery schematics.

Make the Artillery, assign somebody to it, and throw an Artillery smoke grenade in the target location.

Quest stages

 * 1) Return to the Castle
 * 2) Speak to Ronnie Shaw at the Castle
 * 3) Gain access to the Castle's armory
 * 4) Talk to Ronnie Shaw
 * 5) Build and assign artillery at the Castle
 * 6) Talk to Ronnie Shaw
 * 7) Follow Ronnie Shaw
 * 8) Head to the target area
 * 9) (Optional) Keep your radio on and tuned to Radio Freedom
 * 10) Throw a smoke flare into the target area
 * 11) Speak to Preston Garvey

Bugs

 * This quest may cause the transmitter for Radio Freedom to stop broadcasting (Operator complains of lack of power even if ample power is provided).
 * Tuning into Radio Freedom after throwing the smoke grenade may cause the broadcaster to leave his post and refuse to return.
 * The artillery may fail to fire after throwing the smoke grenade, completing all other quest objectives without triggering the "Speak to Preston Garvey" objective, rendering the quest unbeatable.
 * If you start the quest after having already retrieved either the artillery blueprints or smoke grenades (via console commands, etc), you will not be able to progress.
 * Minuteman NPCs dead, unable to set someone to man the artillery.