Another legacy launcher from Fallout 3, the Junk Jet allows you to use any miscellanious junk you've scavenged as ammo. Anything from tin cans to toy cars will give you around 40 points of damage with this machine.
This one requires a quest, so grab the other two and some other weapons first. By walking into Cambridge, a quest called Fire Support will automatically begin. Complete the quest by listening to the new radio frequency and coming to Paladin Danse's aid. After completion, Danse will offer the quest Call to Arms. This quest brings you to Arcjet Systems, where you will eventually find the Junk Jet.