In Doggo and DB we trust
 Arbites Build - Warhammer 40k: Darktide 

I've been trying to make the Double Barrel (DB) work on various classes, and it's passable on Vet and Scum, but I think it really shines on Arb. All three classes have the ability to increase the clip size to three, which is crucial. 

After some trial and error I decided to try combining a variation of my Doggo build with a DB build, along with Terminus Warrant, since it gives you ranged cleave, suppression, and damage, as well as extra reload speed, which is important for the Double Barrel. 

The main drawback of the build is a lack of huge monstrosity damage. Which means you're really reliant on skill to dodge and smack 'em in the head, but with Remote Detonation you can stagger them for yourself and the team. On captains your damage is very solid. Other caveats are that you're less tanky and have less CC compared to a Forceful build, and you don't have the massive boss buffs that Execution Order can let you grab. I also didn't take Rebreather so there's a lack of corruption resistance and no True Grit, so if you mess up and get overheaded, then you're pancaked. But for me that's kind of a skill issue, plus Castigator's can negate it, and if it's a map with the Blight modifier, and you have either a Scum or Zealot, then your corruption worries disappear anyway. 

Went with Up Close for sustain since it branches off of Withering Fire and your ranged kills will regenerate some toughness, while your melee regenerate more, which is enough sustain IMO. It's no toughness on stagger, but sacrifices must be made. The fact that it's talent-heavy means that Castigator's Stance is probably the best Ability option since it requires the fewest additional talents invested to perform well, and it compensates for the slight issue of not having the Arb's usual tankiness with on-demand invulnerability. Plus Canine Morale, and Imposing Force offer quite a lot of DR. Which is almost more useful personally, if not for the team as a whole. It also allows you to occasionally clear a room when the team is pinned down otherwise which does offer some team utility. Plus staying alive allows you to clutch so, still useful. 

I went with Remote Detonation as with a Dog-focused build, I like to take Unleashed Brutality and am more focused on micromanaging the dog than with Lone Wolf, which is usually the better option if you take grenades since you can regenerate them and spam them without being a drain on the team's resources. The main point is that either way you're out of the grenade pool and can funnel them to other teammates. 

So far tested and done work up to Havoc 30. Beaten 40 a few times but not with this build (yet). 

It takes a little bit of practice, and isn't the most ammo-efficient. Plus it takes good positioning due to the DB's short effective range. But with the bleed on crit ability you can still one shot a sniper with a lucky crit from quite far. And if you have a vet, your ammo issues are very low. Add a scum or psyker and it's totally fine. It's only really if there's no vet, and everyone is using a gun (+ no pickpocket scum) that it's not a good choice IMO. 

Anyway, this is pretty fun.