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.