If you have already previously found all weapons, your character will be granted the rarest weapon for their class.
What in the world does this even mean? Do certain classes have a better chance of getting certain weapons? Is it weighted towards beneficial weapons for the class, i.e. Technos get more snipers and fewer shotguns? If so, wouldn't giving the rarest weapon be counterproductive?
Using analytics, we can determine which weapons have been dropped the least throughout all global playtime. So they are the most rare. You'll get one of those.