Ive built every custom weapon and it wont give me it. :nuke: :swear:

What platform are you on?

Xbox 360

I had this same problem with my xbox 360, I didn't receive Scientific Pursuits achievement. Luckily for me on my second play-through I was able to get that achievement, I don't know why because I did everything I did the first time. Sorry I don't have any answers for you.

I know your pain though.

It didnt give the the behemoth achievement and im positive i got them all. i even followed all the notes and carefully avoided the scenic overlook multiple times. i guess it happens.

if you have Broken Steel you can take the perk for unlocking all custom weps. that might fix it. you can always load a save after you get the achievement.

for Scientific Pursuits, i did not receive credit on 360 or on the PC... i started a new char on the PC and followed the main quest exactly and got credit the third try. from my research, my problem was going to the jefferson mem before going to rivet city. seems to break the achievement.