do you have the 'Limited Edition' version?
if so then you need to redeem and activate the freebies that come with it. one or two of them count as 'weapon unlocks' and so, even if you successfully navigate the other issues with this achievement, it won't pop without them.
just redeem the 25-character code in the box and then the next time you go to multiplayer it will pop up a screen offering
to activate or cancel this content. once activated you will see the new item(s) on the weapon unlocks screen.
then either unlock the rest as normal if you have enough tokens or , even if you do, take the cautious route by doing the level 39 glitch for each of the remaining unlocks. should get you the achievement. hope it does.