To get the Xaunlong Assault Rifle you go to the museum of technology.

Go to the terminal on the Ground floor, across the entrance room to start the quest activating a message from Prime telling you that he left you clues to where your loot is if you remeber the old days.  Then select message 001 and select the number 19.   Then go to the terminal In the Museum West Wing, which is past the Vault tour just after entering, seen ahead and a little to the left its the furthest terminal from you and select message 002 and select 53. Then go to the terminal on the ground floor by the Delta rocket and select message 003 and choose the number 113. in case you didnt notice the list of numbers you are given you just choose the prime numbers hence the guys name "Prime".Then you will be give the security office terminal password which gets you get the key to the gun locker in the planetarium office.

You get some weapons and ammo and a note telling you that he will meet you at the diner near the jury street metro station.  Fast travel to the jury street metro station or find it if you havent been there and you will find primes body on a table in the diner and he has the xaunlong assault rifle.

In case you dont know where the jury street metro is it issouth of vault 106 and west of vault 101.