There are two different ways you can get the Junk Jet in Fallout 4, but the first method will be a lot easier if you plan on joining the Brotherhood of Steel. However, if you want to get this weapon without joining the Brotherhood of Steel, there is a random chance of getting the Junk Jet from the Manta Man.
Considered as Fallout 4‘s version of Fallout 3‘s Rocket Launcher, the Junk Jet can be obtained from either one of two ways. The first way is much more easier compared to the second method, if you don’t mind teaming up with the Brotherhood of Steel. However, if you are against this faction, you will want to try to search for the Manta Man to get it.
You will need to go to Jalbert Brother’s Disposal where you will get a transmission on your Pip-Boy. While listening to the frequency, you will get the quest Fire Support, this will lead you to the Police Station which is pretty close by. As you’re approaching the location you will hear shots being fired in the distance, this will lead you to Danse and the Brotherhood of Steel fighting ghouls.
After helping them, talk to Danse to get the quest Call To Arms. This quest will have players follow Danse to the ArcJet Systems and inside the building. Once you are done hacking some computer consoles you will have to fight some enemies through the door opened by the consoles. After splitting from Danse, you will need to go into a room with red lighting; on the desk will be the Junk Jet.
For those having trouble trying to find the weapon during the Call To Arms quest, you can watch MisterNBG walk you through the quest to get the Junk Jet.
Like other customizable weapons in the game, you can mod the Junk Jet, which comes with a base damage of 40, a fire rate of 20, a range of 119, an accuracy of 75, a weight of 29.9 and a value of 285. Lastly, for those seeking to get the Junk Jet from the Manta Man, you will need to complete the Silver Shroud quest, which is given to you by Kent Connolly or by the Silver Shroud radio.
After you are done with the quest, you will need your Luck up high so that you will have a higher chance of bumping into the Manta Man as a Random Encounter.