This gun is optional and can easily be missed. Now I have only beaten the game once so far and I picked the Brotherhood of Steel so im going to base this guide off of that play-through BUT I do know for a fact that you can also do this when siding with the railroad of the institute. At the end of the game before blowing up the institue as the brotherhood a 10 year old Shaun synth will ask if he can come with you instead of being killed in the explosion. This pulls at heart strings but if you pick to leave him in the explosion you automatically forfeit your chance of getting this weapon. If you pick to save synth Shaun he will ask what safehouse to go to, after speaking with the brotherhood and getting your new rank etc you will need to go to the settlement you sent him to and speak with him. He will ask you to get him some materials, I forgot what he asks for a hotplate, some steel and other stuff and after you find all the stuff he needs bring back all the materials and he will craft you the "Wazer Wifle" Its very simple to get but can missed completely. Also its cool to have as this brought back heaps of memories of fallout 3 (: well done Bethesda

Laser rifle, unlimited ammo capacity