I have been playing the Angel of Shotgunnery challenge a bunch of times lately and found the Focused double shotgun to be a really great weapon combined with Shottyman (that i always try to build first).
In Angel of Shotgunnery when you win the Arena special level, instead of an useless rocket launcher (can't use anything that is not shotgun in this challenge), the game gives you a double shotgun, then you only need a Wizkid level and once you run into
power + agility + technical mods
You can obtain this very good upgrade to the double shotgun (more damage, and an alternate fire allowing to shoot only 1 shell, though 1 shell does much less damage) and has a lower spread than the double shotgun, meaning it has a better range before damage reduction (the bad point of the double shotgun is then nullified).
Excellent weapon, the only problem is that shells become rarer and rarer during the game progression (the shells on the ground are only 8 ammo, the only good source is looting the former sergeants for 30 ammo, but unlike in the first half of the game, in the last par, those formers are -very- rare).
And to get the full damage you're going to spend 2 ammos by shots, it can get very fast out of hand for your stock, especially if you have not gone to some Intuition path and then shoot blind around corner to detect monsters.
So if you're going Angel of Shotgunnery and don't find artifacts shotguns, be sure to give a try to assembling the Focused double shotgun, when you're having Shottyman it's a real blast
And if you're a Marine class, withArmy of the Dead it must be really great (shrapnel damage of every shotgun become armor piercing)
Now since the while i am playing 0.997 (and it's a lot) i have not found once a single Nano, Firestorm and Onix , i wonder if the dev made them even more rare, as i have run a lot of character to near endgame in 0.997
I have found a few artifacts but not once a single shotgun ones too.