Old Guns

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

Quick walkthrough

 * 1) Return to The Castle
 * 2) Speak to Ronnie Shaw at the Castle
 * 3) Gain access to the Castle's armoury
 * 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 Artillery smoke grenade into the target area
 * 11) Speak to Preston Garvey

Starting the quest
This Quest can be received from Preston Garvey in Sanctuary at a certain point in the game. Alternatively, a few days after securing the castle, you will be approached and engaged in conversation by a woman named Ronnie Shaw, or by listening to Radio Freedom. She tells you of the old Castle's untapped armoury. After some dialogue, the Old Guns quest will begin, and Ronnie will lead you to a large hydraulic steel door within the Castle, she will tell you that this door is not accessible and you will need to find another way round. Ronnie then leads you inside the Castle's halls into the old General's office where a hallway (now blocked by rubble) leads to the Armoury.

Lower tunnels
Shaw will ask you to clear the rubble. Using the workshop, the Sole Survivor can clear said rubble and gain access to the lower tunnels leading to the armory. This is done simply by scrapping the rubble in workshop mode. Inside are a few rooms with general junk. Also minor defenses, such as mines and a (level dependent?) mk V machine gun turret. Guarding the entrance is a heavy sentry bot called Sarge, who will instantly attack you. After defeating him, you can go into the room before the armoury. Inside you will find a computer with the last general's, General Mcgann's, last entries and also his corpse. You will then need to go through the door to the West Bastion.

West Bastion
Shaw will access a computer that will open the armoury. Inside is an array of plunder, including an artillery schematic and some artillery smoke grenades, which are needed for the next part of the quest. After taking the schematic, it will add an artillery gun to your workshop. Speak to Ronnie, she will ask you to build some artillery guns and assign someone to them. After doing this, speak to Ronnie again and she wants to test the new big guns out.

Testing the Artillery Guns
Shaw asks you to keep the Minuteman radio station, Radio Freedom, on (optional task) and to throw a grenade into the test site. If you have the radio station on, Ronnie will speak to you via it. Once you approach the test site, throw an artillery smoke grenade into the site and step back. After a few seconds the guns will fire and Ronnie will tell you the test was a success. After this, speak to Preston Garvey and complete the quest.

MacCready likes Test Firing the Artillery.

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. (Happens when playing on a different timescale than default 30, set back to normal timescale before throwing grenade and leave as is until "Speak to Preston Garvey" appears.)
 * Deactivating Sarge with robotics perk doesn't trigger next script, you'll need to destroy the robot to proceed.
 * 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. However this can be fixed by adding the smoke grenades to your inventory if you do not have any.
 * Minuteman NPCs dead, unable to set someone to man the artillery.
 * If a settlement has been established, any settlers at The Castle will attack you once you open the West Bastion Gate.