Nothing rips through a crowd of oncoming Feral Ghouls quite like a minigun. Especially a unique minigun with a rare boost that will set those suckers on fire. Yes, on fire. Meet the Ashmaker.
To obtain the Ashmaker, you first need to visit Goodneighbor. Ask around for work and Daisy will direct you to Bobbi No-Nose, a Ghoul who lives at the end of a nearby alley. She will recruit you for a mission and remain elusive about its purpose, but if you pass a few speech checks you can get some clues. She has hired a crew to tunnel under Goodneighbor to nearby Diamond City to raid the storage room. After springing her accomplice from the lockup in Diamond City, you will be ready to proceed.
After assisting her crew with killing Mirelurks along the tunnel expansion (an Eyebot will be using a special frequency to shatter weak walls), you will surface at your final location...Hancock's warehouse. You will be confronted by Fahrenheit and her gang, and be given the option to either walk away or face death. Bobbi No-Nose will need to be convinced through a speech check not to continue, or she will escalate the situation to combat. For siding with Hancock instead of Bobbi, Fahrenheit will reward you with the Ashmaker. It will set your combat targets on fire for 15 points of damage.