Websites for weapon building, starting with my favorite:
1) Larue Tactical Weapon Profiler
All around great looking weapon builder. Has a wide variety of parts, but only Larue Tactical items in the categories of stuff they make (rails etc..) makes sense though.
My one criticism of this is that you have to manually line the items up, there is no “snap” feature. The lack of this feature makes creating ridiculous looking rifles a breeze though.