A quick guide on how to get the Alien Blaster.
MDPL-13 Power Station
Quick Travel to the MDPL-13 power station and walk North along the power lines.
Spaceship crash site
As you walk north along the power lines from the MDPL-13 power station you will start to pick up rads and a radio station after this you will locate a crashed space ship, make your way towards the highlighted area. WARNING! if you have the “Mothership Zeta” DLC going to close to the space ship will trigger your abduction, However, you can use dog meat to retrieve the blaster and ammo without this happening. Having the “Mothership Zeta” DLC also changes the way the Firelance spawns (The FIrelance is a different version of the Alien Blaster) with the DLC it will spawn when you approach the Crash site rather than once you have obtained the Alien Blaster. For details on the Firelance.
Looting the Blaster
Once you’re at the front of the spaceship you will find a dead alien similar in appearance to those featured in the Mother ship Zeta DLC you will find the blaster near the aliens hands and surrounded by ammunition I recommend bringing dog meat to make sure you pick it all up because this is all the ammo you will get until the firelance falls from the sky you will also want to have dog meat with you for this reason because this event could be triggered as soon as the blaster is picked up.
Original Link – Continuation of discussion