you have to stack iron banner engrams and hope and pray you get the new GL or sub machine gun when u turn them in to lord saladin once u have got them u have unlocked them and can now focus them but its rough and u have to get extremely lucky just to get one at all. In my experience its very rare to get any iron banner weapons after completing a match
Good to know that my engram luck is just as bad. Lol I have turned in at least 30 engrams so far and more than half has been armor and no new weapons. Oh well.