BUCKSHOT FOR EVERYONE!
 Hive Scum Build - Warhammer 40k: Darktide 

Huntsman's Shotgun

Dump Stat

• Stability
  This shotgun always recovers from recoil before your next shot, all that sway is kinda cosmetic.
• Mobility
  The most "important" difference between 60 and 80 is increased Sprint Speed - from 0.09 to 0.27. Dodge is practically unaffected.

Perks

• +25% Flak Armoured
  Scabs are everywhere and this allows you to oneshot trash mobs more consistently.
• +25% Maniacs
  With Deathspitter stacked up it allows you to 1-shot bodyshot Ragers. 

Blessings

• Deathspitter  is mandatory - Strength gives you more Damage and Cleave.
• Speedload allows you to pump out more shots during Desperado and drastically reduces your reload time in general.
  
  As for the others:
• Full Bore is impractical most of the time. Its a nice bonus against Reapers and Monstrosities, but nothing more.
• Flechette is just fine. Its great on Veteran, but on Hive Scum you can't do anything fancy.
• Scattershot isn't as versatile as Deathspitter.  It can deal more damage but most of it is overkill and you won't cleave as many targets.
  In theory if paired with Flechette can be great at deleting Elite packs on non-Desperado builds.
  (also don't bother with Vulture's Dodge on this weapon, its not worth it)
• No Respite
  ...
  To be honest I haven't tested it. Maybe worth it, maybe not.

Tactical Axe

Weapon Mark

• Atrox Mk IV Tactical Axe has the best burst damage of all marks which helps you kill Elites&Specials noticeably faster for Pickpocket proc and self defense.
• Atrox Mk VII Tactical Axe has a bit higher DPS then MkIV and so is better against Ogryns and Monstrosities.
• Atrox Mk II Tactical Axe completely loses to other marks. Take it if you want to challenge yourself or smth.

Dump Stat

• Mobility
  Other stats are more important. Simple as.

Perks

I must confess - I did not test taxe breakpoints. I will not act like I know everything anyway and my setup is the right one (its not), so test things yourself or find someone who did that already.

Personally I run:

• +25% Flak Armoured for easier time killing Gunners and flak fodder.
• +25% Unyielding for boss damage.

Blessings

• Brutal Momentum boosts weak spot damage and horde clear.
• Headtaker gives you Strength.
   
• Shred makes Hyper-Critical and Coated Weaponry proc more often. Switch with Brutal Momentum if you wish.

Curios

Why Health - Toughness - Toughness?
You spend most of the time with you ranged weapon out and so can't push or block enemies around you. Health curio allows you to survive more hits without going down.

Fell free to take Toughness - Toughness - Toughness setup if you can comfortably kite and dodge in close quarters.

As for perks, Combat Ability Regen is mandatory. Accatran Mk III Huntsman's Shotgun is extremely ammo hungry, so the faster you get free bullets the better.

For other 2 slots pick what you prefer.

Talents

Voice of Tertium, In Your Face, Speedloader, Pickpocket and Hive City Brawler are mandatory. They are cornerstones of every Desperado build.

Precision Violence helps you survive situations where you can't shoot or generate toughness by shooting (boss fights for example).

Calling for a Time Out has very high uptime with shotguns and you can't go wrong with Toughness Damage Reduction.

Burst of Energy is a safety net. Feel free to swap it for anything else.

Blitzes

Blackout is the weakest yet most versatile of 3. It allows you to quickly make space for retreat/offense or safer revives and staggers Bulwarks.
If you take Boom Bringer or Chem Grenade switch something for Extra Pouches.

Alley Rat makes you dodge too far from your targets and so your Desperado has lower uptime.
Because of that Nimble is a better choice.

Auras

Anarchist is not great for this build since Accatran Mk III Huntsman's Shotgun doesn't benefit from it much. If you want more crit chance take Float Like a Butterfly.
Ruffian is stronger option then Gunslinger Improved for both you and your team. Especially your team.

Ammo Jack adds 1 extra shell in the chamber. More useful then Swift Endurance on this build in my opinion.

Hyper-Critical allows you to oneshot Elites&Specials. If you're lucky.

Punching Above One's Weight is a  really nice bonus damage. Swap it at your own discretion (tho I don't recommend it).

Coated Weaponry helps you kill Ogryns and Monstrosities faster and boosts Sample Collector.

Pocket Toxin is a must take for Blackout. Feel free to swap it if you're using any other blitz.

Keystones

Vulture's Mark doesn't work well with Accatran Mk III Huntsman's Shotgun. It doesn't crit often enough to make use of Vulture's Dodge and its really tedious to keep stacks up while constantly reloading.

Chemical Dependency is a much better option. With Maxed Out Chems you get  ̶4̶0̶%̶  ≈34% Ability Cooldown Reduction from the keystone itself and +20% Crit Chance/TDR from Chem Enhanced/Chem Fortified respectively. Feel free to swap Chem Enhanced to anything else.

Stimm Lab

Go Spur I route into Reflex . +30% Reload speed is great for shotguns.

Do note:
Taking more stimm nodes will screw up Maxed Out Chems timing loop. If you really want to take 30/30 stimm drop Maxed Out Chems.