Desperately Autogun ( Havoc 40 )
 Hive Scum Build - Warhammer 40k: Darktide 

Build alternatives :

1. The Vraks Mk V Infantry Autogun can be built with 60% ammo / 80% mobility, 70% / 70% or how i did it myself. The idea being, ever since the nerf to Pickpocket doesn't allow you to fire infinitly, you will need melee kills every once in a while to sustain your range heavy playstyle. The more ammo you have, the less you have to melee, meanwhile the more mobility you have, the more you can use this agility to position yourself or play aggresivly. Simplified version : Ammo = Damage / Mobility = Safety. Keep in mind, that you can technically regenerate more than 20% ammo with Pickpocket. Killing an enemy once with a melee attack ( not DoT damage ) will regenerate the first 20% ( 116 bullets ), followed by a reload, followed by another melee kill will get you up to 184 bullets total ( 116 in reserve and 67 in magazine ) which is about 31% of your total ammo.
2. You can use both Deathspitter or Dumdum on the Vraks Mk V Infantry Autogun. Both having great damage output and being superior to the other one depending of the situation it is put in.
3. The main uility of the Chirurgeon's Mk IV Bone Saw is downing Scab Captain shields extremely quickly with it's first heavy spam ( 2 hit for one attack ) and dealing decently with carapaced armored enemies without needing to invest talent points. You can keep the Chirurgeon's Mk IV Bone Saw and potentially change the unyielding damage for flack damage or maniacs damage. it'll significantly lower your damage against Scab Captain and twins ( which you mainly deal with your melee ) in exchange for having an easier time against other more common enemies ( thus, procing your Pickpocket that much easier ). Or you could swap it out for the good old Improvised Mk I Shivs lowering a lot your ability to deal with Scab Captain and carapaced armored enemies in exchange for better mobility and better time to kill against any other enemy. 
4. The stamina curio is here to be paired with Hive City Brawler, allowing you to keep for longer a high % of damage reduction and allow longer mouvement. If you prefer more raw survivability, swap it out for a Toughness curio. Keep in mind that one of the main way for a hive scum to survive is it's mobility, depending of how good you are at using it, mobility is more valuable than Toughness ( to an extent of course ).
5. Blackout grenades became an extremely strong option ever since the introduction of Pocket Toxin. The idea of being able to regenerate basically infinitly grenades that can one shot 70+ enemies when well placed is extremely appealing ( 3 stack of chem toxin being enough to wipe out even Moebian 21st enemies. ). It'll also do significant damage to any and every elites in the game while staggering everything it touched on a large radius ( apart from monstrosities ). Finally, it'll keep the grenade economy of your team extremely high, allowing for Frag Bomb spam from an ogryn or grenade usage from a Zealot. If your team lack carapaced armored damage, you are the only one using grenades or simply you aren't a fan of the Blackout then swap it for Boom Bringer. If you do so Pocket Toxin become unecessary and you can take ether Street Tough ( much stronger survivability perk ), Swift Endurance ( easier melee time also making the Improvised Mk III Shivs much stronger ) or take Blessed Stimms and swap Gunslinger Improved for Anarchist ( lowering the ammo economy of the team but boosting your own damage significantly ). ( WARNING ! - the Blackout grenades regeneration only work with direct kills, not DoT kills, which means that all kills done by Toxin or bleed won't count ! Do not expecate to regenerate one after trowing one and killing 20 enemies. )
6. Nimble and Alley Rat are interchangeable here, both give great value. I prefer Nimble simply due to not needing the space Alley Rat would require. Yet Alley Rat allow you to maintain distance much easier if you are in need of it. Pick what you love !
7. Your stimm have many different ways ot being used. You could go for only one the Wildfire I branch, making your Stimm 45s Cooldown, allowing you to spamm finesse damage a lot more. You could go for the one shown above, giving you less stimms for more Enhanced Desperado, significantly boosting your damage. You could go for both the Wildfire I branch and the Barrage I branch ( until Regain , Tank  being significantly better on melee builds for very little impact on range builds ), giving you a very strong survival tool on a 75s Cooldown. it's a matter of preference, but all of those combination are good in this build.

Strength :

• Your raw damage output is as important ( if not more ) than a Rifthaven Mk II Inferno Force Staff Psyker. Hordes go down without any issue between your gun play and Blackout grenades ( even against Moebian 21st ). Monstrosities can be handled easily with constant weakspot range hits. All elites apart from Crusher can be killed with gun play again ( Crusher being handled by ether Blackout, Chirurgeon's Mk IV Bone Saw or your team. Scab Mauler being handled by torso shots ). Scab Captain and Twins being handled by your Chirurgeon's Mk IV Bone Saw to easily go trough shield, do very high damage with heavy attacks AND apply Toxin on hit. 
• Both your ammo and grenade economy are amazing. You do not need to pick basically anything apart from medical crate and ammo crate. On top of that, Gunslinger Improved will make sure to maximize each ammo pick up of your team, making ether Survivalist sustain even stronger, or even not needing a Veteran for ammo regeneration !
• You are extremely mobile, allowing you to save teammates easily ( paired with the stagger from Blackout ), play aggresively, or simply reposition any time needed. 
• Technically, as you can handle Hordes so well, you allow psyckers to play very differently, or even allow team compositions without any psyker to begin with !
• Between Voice of Tertium, Burst of Energy, Tis but a Scratch, Enhanced Desperado range damage immunity and Hive City Brawler, you'll have very good survivability at all times. Meanwhile Nimble and Vulture's Mark guarantee high mobility making you that much harder to kill.
• There isn't a single position you would be uncomfortable with. You can handle extremely tight space as good as you would handle long range. 

Weakness :

• Most of the time, the Hive scum is considered as a heavy melee damage dealer equiped with Boom Bringer for tougher situations or armor spams. Here, you are an extremely verstatile high damage range dealer that bring Blackout making your carapace armored damage much lower than expected. This will need to be taken into account when building your team. 
• You are still a hive scum, at the end of the day, 2 hits from the most basic enemy and you are down if not dead ( in Havoc 40 ! ). Making the skill floor higher than most class / builds. 
• Ever since the nerf to Pickpocket, you cannot afford to simply gun all the way trough a game, you will have moments without ammo, and you will have moment without ammo that happen in very tough situations. Knowing how to pick a melee kill in those moments or/and being aware of the cooldown of Enhanced Desperado will be a make or break for you.
• You cannot play against Rotten Armour, none of your tools being strong enough to deal with buffed up armored enemies all the time. 

Conclusion :

This build can deal with everything that can be thrown at you apart from raw armor spam ( Rotten armor being the worst ). Of course you will be able to deal with normal armor spam, but not nearly as good as the usual Hive scum build would. 
Similarly to the other Hive scum build i wrote, the power of this build is directly related to how good you are as a player. The better you are, the more strength you will be able to get out of this build. 
Ever since Pickpocket changes, Gun Hive scum have basically been taken out of the havoc meta, this build brings back an extremely strong form of it, which, to me at least, is one of the most fun thing there is ! ( less seen = more fun ahaha )

A build that is true to the insane strength that the Hive scum can bring while keeping it unique !
A build that allow you to boost a otherwise dead weapon to the top of the meta ! ( A bullet weapon as that ! ) 

A build that isn't Meta but that is as fun as it gets !

Worth learning ! Worth playing at every level of play ! Worth the fun !

I encourage you to try it out and tell me what you think ! ;) ( Giselle5957 on discord ;) )