You get to order a weapon every rank. You get more options to choose from when you get your weapon on arms day. Supposedly the higher the rank the better the roll will be. You get a package every rank as well that contains a legendary weapon. Quests for legendary and exotic weapons at level 2 and 3 respectively. Think that's enough to grind it out. I usually do all weapons on all characters on Wednesday. Takes between 10 and 30 minutes per character depending on what exactly needs to be done.