Levelling up guns was a completely useless system. It was just a pointless additional step you had to take when you got a new gun to unlock the perks. All everyone ever did with a new gun was equip it, go into the director and consume a whole bunch of motes of light to level it up. Let's be honest no one ever levelled guns up by just playing the game. Removing the need to do this every time was a good decision.