Its blind luck as well. I don't really use shotguns much so invective isn't useful to me. You want as many as you can because there are 3 types of weapon, 3 sub types and then when you add in all the different elemental effects you have a lot of weapons that would be useful. Now that damage isn't really tied to the rarity of weapons its not such an issue that you can only equip one exotic at a time either.