There are ways of rank up your weapons and armor in Destiny apart from shooting lots of enemies at a time.
Using Bounties to Quickly Rank Up Weapons or Armor
You need to equip the gear you want to rank up, then turn in completed bounties to get instant infusion of XP added to your character level progress and to any gear (onlu gear equiped gets the XP reward when turning in Bounties). The XP is shared between gear (requiring less XP to rank up) and progression.
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