Plague Marine on Adderall: A Needle Pistol Study
 Hive Scum Build - Warhammer 40k: Darktide 

This is a Needle Pistol build, but before I explain it, you need to learn a few things about this weapon that the game doesn't tell you.

First, some basic Needle Pistol info:  Toxin types and weapon marks

There are two marks available and both use yellow and blue toxins. Yellow is the same for both, blue is not. 

    • Yellow Toxin: It does very high single target damage and ignores armor. It may not be as devastating as a krak nade, but's definitely fun. Just know that at higher difficulties, you'll need to unload pretty much an entire mag, and that it'll take about four seconds to kill. That's long enough for a Crusher to attack at least once (meaning death), but it's easy enough to dodge/sprint/slide out of range. 

          • NOTE: There is a damage cap on dots (like 600 something on the first 8 ticks), so don't just constantly shoot at a single enemy. 

    • Mk VI Blue Toxin - Explode Strain: Causes enemies to explode on death, doing pitiful AOE damage to nearby enemies. The AOE explosion damage is so low that I have to assume it's bugged. If that weren't bad enough, it doesn't do any stagger either, further making me think it's bugged. For now, use the Mk II.

    • Mk II Blue Toxin - Area Dispersal Strain: Causes enemies to explode on death, spreading a slightly weaker dot to nearby enemies. Enemies killed while affected by this toxin will spread a slightly weaker dot to surrounding enemies. You know it's working if you see enemies explode into a little ball of green smoke. The blue toxin is primarily used as a horde killer but dont sleep on its single target damage. Switching modes in the heat of the moment can be tricky, so feel free to use it against pretty much anything else.

    • Horde strategy note: Try to limit your shots to one needle per enemy . Empty the clip in a horizontal motion across the horde to ensure maximum toxin spread.

    • A note on Bulwarks: Both toxins from both marks of Needle Pistol have the amazing ability to bypass armor. Yes, even carapace. However, they cannot penetrate the Bulwark's shield. The sound effect from hitting carapace is the same *ding* sound as when you hit a Bulwark's shield. This can be deceiving because the Bulwark wears carapace armor on his arms and lower legs (the rest of him is the Unyielding armor type). 
 

Now that the basics are out of the way, here are some important things to know about this weapon that you need to know before using it.

Needle Pistol Things to know #1:  Critical hits don't work like you might expect.

1. It can only crit off the initial hit. As in, the actual needle puncture. The dots themselves do not crit and thus cannot be used to proc another ability that requires a crit. This also means you get 6 chances to crit per reload. 7 with the Ammo Jack talent. Now you might be thinking "yeah but the revolver only gets 5 chances to crit per reload and it's awesome". Well yeah, that's because the revolver is, in fact awesome. The Needle Pistol is too, but isn't something you want to make a crit build out of, due to what actually happens when it crits. Which leads me to...
2. Critting with the Needle Pistol does not increase the damage of each the dot. If the initial hit from a Needle Pistol actually scores a crit, it doesn't mean the dot will do more damage. Instead of that, the dot's lifeline is extended by a whole two extra ticks of damage. That's bad enough but it gets worse. Here's a quick rundown on how the numbers work:
   1. Let's say and enemy is affected with a single stack of chem toxin.
   2. The first 8 ticks of the dot hit for the same amount of damage and are also the highest damaging. After that, the damage drops off with each tick. It works the same for both yellow and blue chem toxin dots.
   3. When you crit with the Needle Pistol, 2 additional ticks of damage are added to the dot, but these 2 ticks are not added to the first 8. They're shoved in somewhere on the way out, meaning they don't even do good damage.
3. Weakspot hits DO increase DOT damage, but only for the yellow toxin. I get why they designed it like this, since blue is for hordes and yellow is for single targets.
   1. Still testing note: I don't think the dot damage increase from hitting a weakspot stacks. Either that or there's an invisible cooldown that happens before you can get the bonus again.

Needle Pistol Things to know #2:  Some Blessings don't work like you might expect.

• Speedload
       What it says: +10% Reload Speed for 3 seconds after Close Kill. Stacks 5 times.
       What it doesn't say: Can only proc off of the initial needle hit, not the dots.
• Gloryhunter
       What it says: 25% Toughness on elite kill.
       What it doesn't say: Can only proc off of the initial needle hit, not dots.
• Desperado
       What it says: 20% crit chance for 6 seconds on successful dodge.
       What it doesn't say: The extra crit chance can't be applied to melee weapons.
• Concentrated Fire
       What it says: Up to +5% Critical Chance on Chained Ranged Weak Spot Hit (Any Target).
       What it doesn't say: Only procs off of the initial needle hit, not the dots.
• Point Blank
       What it says: +20% Ranged Critical Chance after Melee Kill for 3.5 seconds.
       What it doesn't say: It doesn't tell you how crits work on this weapon.

Needle Pistol Things to know #3:  Some Talents don't work like you might expect, either.

• Speedloader 
      What it says: +30% Reload Speed for 8s on Close Ranged Kill.
      What it doesn't say: Same as the blessing. Can only proc off of the initial needle hit, not the dots.
• Unload
      What it says: Reloading your Ranged Weapon grants +2% Ranged Damage for 7s. Each 10% of magazine spent during the duration grants +2% additional Ranged Damage.
      What it doesn't say: The ranged damage boost only affects the initial hit, not the dots.
• Nimble
      What it says: Dodges are extended by +25%, and time considered Dodging by +50%.
      What it doesn't say: I'm 99% sure this is bugged. In fact in my testing, it feels like the opposite is happening.
• Swift Endurance
      What it says: +2% increased Melee Attack Speed for each current Stamina.
      What it doesn't say: Caps at 10 stacks. Only applies to available stamina.
• In Your Face
      What it says: +25% Ranged Damage against targets within 12.5 meters.
      What it doesn't say: The ranged damage boost only affects the initial hit, not the dots.

And here's some talents I'm still testing:

• Voice of Tertium
      What it says: Replenish +8% Toughness on Ranged Kill. Elites and Specials instead replenish +15% Toughness.
      What it doesn't say: While I haven't tested this yet, I assume it acts the same as everything else that procs from "ranged kills", ie. the needle hit, not the dots.
• Hyper-Critical
      What it says: Critical Strikes instantly kill Human Sized Enemies if their current Health is less than 2 times the amount of Damage of the Critical Strike.
      What it doesn't say: As I'm still testing this, given the previous info I have to assume it only procs off of the initial needle hit, not the dots. Seeing as how the initial needle hit does such tiny damage, I reckon the chances of this proccing with the Needle Gun seems very slim.

Melee Weapon - Shivs

I prefer the Shivs but its your preference really. Just make sure to use something fast that does good crit damage. Why is that so important? Deep breath…

More attacks per second means more crits per second.
More crits per second means more kills per second.
More kills per second means your Cartel Special Stimm as a lower cooldown (Sample Collector talent - lowers stimm cooldown per kill).
A faster Cartel Special Stimm cooldown means a faster Stimm Supply ability cooldown (Practiced Deployment talent - Stimm Supply comes off cooldown when your cartel stimm does).
A faster Stimm Supply ability cooldown means you’ll have a much easier time maintaining 4 stacks of Chemical Dependence.
And finally, being able to maintain 4 stacks of Chemical Dependence means longer uptime of your Chem Enhanced and Chem Fortified buffs (5% bonus to crit and toughness damage reduction per stack of Chemical Dependence, respectively).

Oh, and you can throw them. Useful in a pinch, if you're out of ammo and the only elite or specialist enemy too far for melee (Pickpocket talent).

Shiv Blessings:
    • Uncanny Strike - It's great on the Combat Blade and it's great on the Shivs.
    • Riposte - Being a good Hive Scum means you're going to be dodging a lot. Like a lot lot. Thankfully, there are several talents that enhance your dodging and all of that makes this blessing a no-brainer.

Shiv Perks:
    • 25% Unarmored Damage
    • 25% Flak Damage

Tbh, you can run whichever perks you want. 
 

Curios

     Blessings
          • 3x Max Stamina
          • Two Health Bonus - I see a lot of Hive Scum running Toughness curios, but in my experience, I find that I stay alive much easier with more health.

     Perks
          • Stamina regen - Synergizes well with the Needle Pistol Stripped Down blessing, which makes you invulnerable to ranged damage while sprinting at over 50% stamina.
          • Sprint Efficiency - See above.
          • Health Bonus (but this is negotiable). 
 

A few notes on talents

    • Sample Collector: This talent is essential to this build and is the reason why you want to stack crit strictly for melee purposes. More melee crits means you can kill faster, which means your stimm cooldown (not the ability, the stimm) goes away faster, which means you can keep four stacks of Chemical Dependence up easier.
          - BIG BIG NOTE: If you are stimming yourself by using the Cartel Special Stimm, and not the Stimm Supply ability, then Sample Collector only kicks in after the cartel stimm effects have worn off, which is a whole 20 second delay. HOWEVER, if you're getting your stimm effects from the Stimm Supply ability, which you should be, then this talent works immediately with no delay. I assume this is either a bug or just a bad design choice, and I hope at some point they'll get rid of the 20 second delay when using the cartel stimm.

    • Regained Posture – Only get this if you're using the Stripped Down blessing, which you should be, and even then it's optional at best.

    • Sticky Hands - Synergizes extremely well with both Stripped Down and Hit and Run blessings, allowing you to dot up more enemies with chem toxin, as you sprint through the area immune to ranged damage. Just keep your eye on your stamina bar and don't forget that even Gunners have melee attacks!

    • Slippery Customer - FINALLY, you can dodge melee attacks by sliding. I can't tell you how much I love this talent, and how many times it's gotten me killed when I play other classes and forget they don't have it.

    • Hive City Brawler first, then switch to Pickpocket – If you’re having issues staying alive, I would recommend going with Hive City Brawler for the increased damage reduction. Just make sure to stack some stamina. Once you've got a handle on things, switch to Pickpocket asap. The sheer amount of enemies in Damnation and Auric difficulties pretty much make this a required talent, due to how easy it is to run out of ammo.
 

Blitz

     Chem Grenade – All of the blitzes are great but I prefer this one. The Chem Grenade works wonders as area denial, and it's the main reason why I prefer it over the bazooka. You can either use your bazooka and kill a bunch of enemies in an instant, or you can drop a chem grenade and then forget about it, allowing you to focus on something else, all the while the chem nade is killing an entire horde. Also, if you’re working a device or trying to revive/rescue a teammate, and you keep getting interrupted, toss one down at your feet. Weaker enemies will die before they get to you, giving you enough time to finish the task.
    • NOTE: Chem Grenades do not stack. Don't throw two at once thinking it will hit twice as hard because it won't. Only use more than one if you want to continue the effect, and even then be sure to time it so the second one goes off at the end of the first one.

Aura

     Gunslinger Improved or Anarchist: If you're good about maintaining your ammo, then Anarchist all day long. If not, Gunslinger.
 

Ability

     Stimm Supply – 20 seconds of corruption healing (as long as the corruption hasn't covered a full wound bar) and cartel special stimm buffs, for you and your team.

          • Modifier: Fast Acting Stimms – Must have. Stimm Supply only lasts for 5 seconds but the effects remain for 15 seconds so there’s no penalty. And as fast as this game is, no one should be expected to stand near the Stimm Supply for 20 seconds. I imagine this talent will just get merged into the main ability at some point.

          • Modifier: Practiced Deployment - Effectively merges your Cartel Special Stimm and Stimm Supply cooldown into one. As long as you get Sample Collector (reduces Cartel Special Stimm cooldown for each kill) further down in the tree, you’ll be able to use Stimm Supply incredibly fast. Sometimes it’s almost instantly.
 

Keystone

     Chemical Dependency – This build is fun, but it does require you to always be aware of a few things on your HUD. You need to keep your eye on how many stacks of Chemical Dependency you're at, your Stimm Supply cooldown, and your Cartel Special Stimm cooldown. The big reason why is because there will be times when your Stimm Supply ability comes off cooldown but your Cartel Special Stimm is still on cooldown. You need to wait until the Cartel Special comes off cooldown before using Stimm Supply, if you want your stimm's effects and not just the corruption healing.
          • Note: You can use Stimm Supply ability when the Cartel Special Stimm is still on cooldown when you need to extend the Chemical Dependency buffs. Then you can use it again once your stimm is ready. The only problem with this is it can be confusing for your team because both use the same visual effects. When your stimm is ready, try to use Stimm Supply when everyone is grouped up.

          • Keystone Modifier: Chem Enhanced – A shot of 20% extra crit chance (for melee)

          • Keystone Modifier: Chem Fortified – Dropped in a glass of 20% toughness damage reduction, garnished with a sprig of 50% toughness replenish, and you got a hell of a  stimm cocktail.

          • Keystone Modifier: Maxed Out Chems – The Chemical Dependency stacks drop off one at a time so there’s not reason not to get this.
 

Stimm Lab

Strength branch for the Crit bonus at the end. After that I advise getting the Toughness branch for team survivability. You can run one of the other two if you want but I feel like you're kind of messing with your team if you do. They can't see what buffs they're getting, and I'm pretty sure most people expect at least the toughness buff from a Stimm Supply.