My best advice to you to level up to 20 quick is to just go through the story mode. After each mission, return to the tower to check for bounties. Do all the story mode strikes along the way as well. Once you hit level 20, buy the Future War Cult bond for your Warlock, and level them up. Wear that bond when you complete strikes and turn in bounties. Future War Cult has a chest armor that increases the amount of ammo you can carry for hand cannons AND sniper rifles... I know because it's the exact armor I use! You will need to level up with FWC to level 3 to get that armor though (because you will need a Crucible commendation in order to purchase it).