While the other guides are fantastic for detailing where each weapon is found, after 2 playthroughs I found myself having missed something along the way. I found this walkthrough that helped me. You have to do it all in one playthrough and you can't skip levels.http://metro2033.wikia.com/wiki/Weaponsmith
Start at Armory and work your way to D6. One thing I noticed is that I had a hard time finding a Duplet in Frontline, so I waited until later.
This should be obvious, but I'll say it anyway... make sure that you have the DLC downloaded before you start. I bought the DLC but didn't have it downloaded on my main console at first, so my early kills didn't count.