To get the secret rocket launcher "Annabelle" you need to go to Neils shack and do his quest (Neils shack is located at the bottom of black mountain).

As you are going up the mountain you will run into a couple of Night-kin and some super mutants. After you kill them there should be a radio tower and a Night-kin sniper using a rocket launcher.

After you kill him go up the tower and take his rocket launcher.

Also if you check all the building you can get Raul as a companion and do a scavenger hunt inside a supply room.

This will let you find a missie launcher, a couple of grenades and some C4.