You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
From what I understand, even before you unlock the challenge, the game still registers those long-range shotgun kills while you complete Open Wide. So you don't have to start from scratch once unlocked.
Most Hyperion shotguns should work fine.
It's raining luggage and babies and limbs and Daddy doesn't come home.