You don't have to use all the weapons on the list, I only listed some locations for most of the weapons (yes, they do appear in other chatpers than the ones I have listed like the small pipes and conduits everywhere).
If you've already played through a chapter, you can go back to that chapter and replay it if you missed a weapon and it will count towards the achievement. Just make sure you don't restart a chapter after completing it or you might run into that bird bug that others have been talking about on this forum.
The checkpoint save system can be quite annoying though so if you plan on saving, make sure that you reach a particular checkpoint. The last thing you want is to find that your game wasn't saved (which has happened to me). I like to run through a chapter in one sitting though.....just for closure to it.
Always glad to be helpful and answer what questions I can about the game....
informer