REQ Pack Question. Pls help.

So I know that getting the good stuff in req packs is kinda hard but my questions are.

If you want to aim for legendary weapons do you have to unlock that certain weapons common, uncommon, and rare certifications card first or is it possible to unlock the legendary weapon before unlocking its uncommon, rare certification? Like the nornfang for example its a legendary sniper do you have the chance in unlocking that before the arrow of time sniper which is like rare or ultra rare?