Actually from what I have seen, they scrapped the idea of interchangeable parts and instead are now having the weapons drop random items when dismantled. There is "spinmetal", "weapon parts", and "helium canisters". These are what were in the beta at least (I reviewed legendary weapon upgrade reqs too so don't think it's because of low tier weaponry). This means you won't be able to put one really cool scope from a gun unto another gun. That being said, from what I saw, the upper tier weapons included ALL of the scopes found from other weapons in their class. For example if there was an uncommon shotgun with a trusight, then all legendary weapons would have an upgrade for trusight available. I think what's most important to note from this is that while all of the various types of scopes will be available no matter which legendary you pick up, there are aesthetically different trusights (for example). On one fusion rifle I got a red trusight on another I got a completely different looking trusight that was green. What's important to also note is that they weren't different fusion rifles. Instead they were the same exact fusion rifles (name and all) except the aesthetics of the sights. So basically, guns will be like pokemon. If you want that shiny dragonite (or in this case that green sight) then you will need to keep catching them in hopes of getting it (or in this case keep killing monsters or running strikes/raids in hopes of getting your favorite sight).