In the IH on pgs 245-248 it outlines building things in Dark Heresy. But this seems a little too broad somewhat. How would you handle someone who wants to try to build something like a plasma gun or something that is very rare? Now this is given that they have all of the time and money needed to do so.
Now I am to assume that having the appropriate Trade skill would give you the ability to build more common items or gear. But where would you draw the line?