In case you didn't know, the most important feature of a shoot to loot gun is that it lets you basically fill your entire heavy reserve ammo by shooting the same ammo pack multiple times. The pack has a despawn timer of about 2 seconds that starts after you shoot the ammo pack for the first time. Continued shots before it despawns will continue to give you ammo. This is incredibly useful in CE hard mode. 2 different gun types have the "shoot to loot" perk on them, scout rifles and sniper rifles. They can be found in roc strikes, the crucible, and occasionally at the gunsmith. They are green or blue quality only, no legendaries. The scout rifle is preferable for 2 reasons. One, it is much easier to get consecutive shots on the same ammo pack, and two, it's fairly safe to leave it equipped for the duration of the Crota fight (after your team clears out the first room). There's not a whole lot that you'll need to kill with your primary, but having your favorite sniper at the ready is fairly important. Not to mention, if it's a blue scout rifle, you'll be able to get 297 damage when it's maxed (almost the damage of VoC). Finally, finding a "shoot to loot" scout rifle that has exploding rounds on it is like gold. You'll be able to loot all ammo that the explosion touches and it's much easier to land consecutive shots on the same ammo pack from far away. If you're worried about accidentally deleting yours because of it's color, make sure to fully upgrade it so that it has a yellow outline around it. Another tip is to move it around in the item slot to a location in the middle of all your legendaries. You'll be less likely to screw up. Good luck!