Old Guns

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

Detailed walkthrough
Approximately three days after the Minutemen retake the Castle, Ronnie Shaw will arrive there seeking out the Sole Survivor. Preston Garvey will inform the Sole Survivor that the Castle has been trying to reach them via radio. Tuning in to Radio Freedom will play a scene where Ronnie tells the Sole Survivor to "get your heinie to the Castle pronto."

To begin the quest the Sole Survivor must travel to the Castle and talk to Ronnie (the scenes with Preston and Radio Freedom are optional). After speaking about her prior involvement with the Minutemen, Ronnie will inform the Sole Survivor of an armory hidden in the west bastion of the Castle that only she knows how to access. She will then lead them to the steel door on the west wall of the Castle. The door cannot be opened from the outside. After talking to Ronnie again, she will lead the Sole Survivor to an alternate entrance: a tunnel in the general's quarters in the east corner of the Castle walls. The tunnel will be blocked by some rubble, which is easily cleared by scrapping it via the Castle workshop. With the rubble cleared, Ronnie will lead the Sole Survivor into the tunnels.

Inside the tunnels the duo will find a maze of storage rooms with some miscellaneous loot and a few minor hazards: an automated turret, some environmental radiation, and a handful of frag mines scattered about. A little exploration will reveal a shelf of cannonballs for the Broadsider and a generator containing a fusion core.

As the duo approaches a security gate at the other end of the tunnel, they will encounter a Sentry Bot known as Sarge. A short dialogue sequence will reveal that Sarge has been guarding the Castle for ages, but has unfortunately gone haywire and is now hostile. Once the bot has been neutralized, Ronnie will access the terminal on the desk and unlock the security gate. General McGann's corpse can be found in the next room, alongside a crate of wine and a terminal detailing his fate. Ronnie will inform the Sole Survivor that McGann's uniform is theirs to take as the new general, before leading onwards to the west bastion.

After climbing the stairs out of the tunnel, the duo will find themselves on the other side of the steel door. Ronnie will open the door using the nearby button to verify that it still works, then lead the Sole Survivor into the armory. Inside the armory, they will find several weapon mods, a Fat Man, some mines, turrets, traps, and other munitions, and the ultimate goal of the quest: schematics and materials to build artillery at settlements. Ronnie will tell the Sole Survivor to grab the schematics (as well as some smoke grenades) and then build some artillery to do a test fire.

At this point, the Sole Survivor will need to use the Castle's workshop to build an artillery piece and assign a settler to operate it. With the artillery operational, Ronnie will give a rundown of how it works before naming a nearby building as the test area. The Sole Survivor can then proceed to the test area, tune in to Radio Freedom, and toss a smoke grenade to call in the test strike. Once the Sole Survivor has moved out of range, the artillery will bombard the area. Ronnie will then call out their success over the radio, confirming that the Sole Survivor now has the ability to call in artillery support (and admonishing them to "use it wisely"). With the test successful, the Sole Survivor can report back to Preston to complete the quest.

From here, the Minutemen quest line reaches a stopping point. It can be continued by becoming hostile with the Institute, leading to Form Ranks (or directly to Defend the Castle if the Minutemen have at least 8 allied settlements). Alternatively, it can be continued with With Our Powers Combined by destroying the Institute with the Brotherhood of Steel and then becoming hostile with the Brotherhood.

Bugs

 * Deactivating Sarge with the Robotics Expert perk does not trigger the next script, therefore the player character needs to destroy the robot to proceed or on PC open console and type . If Ronnie still does not react set the stage to 480 to open the door.
 * If starting the quest after having already retrieved either the artillery blueprints or smoke grenades (via console commands, etc), one will not be able to progress. However, this can be fixed by adding the smoke grenades to the inventory if not having any. This can be done with the console command . If the quest still does not proceed, teleport through the wall by getting the door open or toggling noclip and grab the blueprint from the table.
 * Ronnie Shaw may get stuck "taking a knee" if knocked out during the fight with Sarge, preventing her from progressing the quest. Going back upstairs and returning, or sleeping in a bed in the room seems to reset her and allows her to work on the terminal.
 * Ronnie stops being a vendor and gets locked in "trade mode" when talked to even when she is in the armory. Seems to happen at random.
 * Tuning into Radio Freedom after throwing the smoke grenade may cause the broadcaster to leave his post and refuse to return. To fix:
 * Open up console using ~
 * Select the radio transmitter (hex value should be )
 * Type:.
 * Assign any settler to the radio transmitter; this should force the Minuteman to sit down and start using it again.
 * Alternatively, if he still will not return to his post, the following command will force the radio to start broadcasting without him:
 * 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. Applying the radio-fix above, and then throwing another smoke grenade to the same area as earlier in the quest, can trigger the artillery, and the quest will continue. Another possible fix is to use a command to modify the timescale:
 * Open up console using ~
 * Type:.
 * Wait and see if the artillery is firing and if the quest can go on.
 * Revert back to timescale default value, type:.
 * The radio operator may complain that he can not use the radio as it needs more power even though the required amount of power is provided. He then refuses to sit at the radio and no other NPC can be assigned to the task. If it occurs before this quest is completed, it renders this quest uncompleteable. Could be linked with Ronnie Shaw suddenly ceasing to be a vendor.
 * Ronnie gets stuck behind the door that the computer terminal needs to open and the terminal says inaccessible. This happens when Ronnie takes a knee in the hallway prior to entering the room with the sentry bot. When a bottle cap mine is placed at the right corner of the room's doorway and the the mine goes off (triggered by the sentry bot), Ronnie disappears and the sentry bot gets stuck. Once the bot is defeated, Ronnie appears behind the security door and cannot come through to unlock the terminal.
 * When doing this quest, once underground in the Castle's armory, there is a chance Ronnie Shaw can die and spawn in the other side of the room preventing her from opening the door that only she is able to open, which is game breaking as it prevents completion of the quest.
 * To fix, move Ronnie to the player character with.
 * When Ronnie leads the player character outside for a test firing after building the artillery and collecting the smoke grenades, if extensive work had been done on the walls and/or outside prior or before this quest, Ronnie will continually attempt to reach the location needed for the test fire, but the quest will never progress.
 * To fix, progress the stage manually with.
 * Sometimes the Castle Armory door will break (most often it is due to opening the door before its intended activation.) This can cause it to get stuck in either an open or closed state, causing the button on the interior of the Castle Armory section to stop working during this quest.
 * To fix, open the console, click on the door, then and . This should fix the issue and reset the door to its correct state.