First off, in order to do this you will need to have the ability to pick master locks. Then, you will want to fast travel or walk over the location called the “Student Union Building.” It is located on the east side of the map around the center of the east coast.
Once there fight off any enemies that are in the area and head toward the building. If you see the remains of some dining booths, you are in the right area. Go inside and head right, you should see a terminal next to a large steel door. It will be on the first floor.
Hack the terminal and go inside the door. Once inside, head down the stairs and hack yet another terminal. This is all worth it, I promise.
Once you have hacked the terminal and opened the vault door, head inside. Here you should see a bunch of safes. Most have been opened or have the doors missing. Look for the safe that is still locked. This is where your master lock picker skills come in.
Pick the lock and open the door. Inside you will find a red button on the right side of the safe wall. This is a good red button – not an end of the world red button – so go ahead and press it. Look to your left and the wall panel should open up. Inside will be the gun and some ammo.
Ta-da! Never reload this gun again, Reloading is for noobs.
Map image source
Safe image source
Gun image source