Captain Blast; Mk III Boltgun Rise Up Build
 Arbites Build - Warhammer 40k: Darktide 

This build was a personal challenge to make Boltgun Mk III, Castigator's Stance and Terminus Warrant keystone good in endgame (Havoc 40) since 'Bound by Duty' update; initial results were satisfying despite clear limitations and eventually the build was refined.
Since 'Beyond The Hive' update however and buffs to Castigator's Stance and changes to Terminus Warrant the build required a refresh... and in the end became even more fun and powerful! Can say with full confidence that this build is my crown jewel of build crafting!
Now; unto the important stuff!
TIPS:
1. The obvious star of the show is the Boltgun Mk III. Since release however it was... not well received; designed to be a crowd killer rather than a single target killer like Boltgun Mk IIb paired with low ammo count, slow reload speed (despite being faster than the original mark), same swap speed and slower firerate quickly put it in the bin by the community...
However... its potential is massive; if build properly it turns from an underwhelming tool to a killing machine as it rightfully deserves! Just like a jewel requires polishing to luster so does the choices of perks and blessings matter for it to shine;
- +25% damage againts Flak and Unyielding offers perfect balance in killing potential; Flak covers majority of the enemy roster while Unyielding helps in chewing Bulwarks and Reapers massive HP pools
- Pinning Fire is a no brainer choice; the weapon staggers very well on its own and radius ensures even the more distant chaf enemies will be affected and thus buff consequtive shots direct and splash damage
- Fire Frenzy is a surprising discovery; despite being Close Range damage and not Power or Strength like Pinning Fire its still up to +50% extra damage in range where most of the encounters happen which is 12,5 meters; when beyond that it still provides alot despite diminishing returns! It buffs both direct and splash damage too which in turn makes crowd killing faster and better!
Despite many other good blessings the two listed above are Your best choices; the rest are worse in one way or the other;
- Cavalcade works only on direct hits; the splash damage cannot crit (atleast on this weapon) and thus its not worth using
- Gloryhunter offers nice toughness restore but its not required in the slightest
- Inspiring Barrage offers nice toughness restore but its not required in the slightest
- Puncture works only on direct hits and despite being very good its not better than the other options (if it would work on splash it would be overpowered; EVERY build would use it cause effectively You would have an equivalent of a mini Shredder Frag Grenade from Veteran!)
- Shattering Impact works only on direct hits and thus its not worth using (if it would work on splash it would be overpowered; tried using it to do damage to Crushers but it was not worth it in comparison to other options)
- Surgical is nice for sniping and You will sometimes have to but overall it is not better than other options (even Mk IIb does not use it and its more of a single target oriented weapon than the Mk III which should paint the picture already)
- Terrifying Barrage does what the weapon already does to begin with and does it better with added stagger and killing potential in a radius
2. You can use Shock Mines instead of Arbites Grenades however IMO Arbites Grenades are better due to damage, reactiveness and Boss stagger capabilities
3. Your melee option should be able to keep You alive while also serve as a crowd control tool with an added edge againts Carapace. Arbites Shock Maul fits all of the criteria above with the correct combination of perks and blessings;
- +25% damage againts Carapace and Unyielding maximizes damage againts the most beefy targets; it is better to amplify the damage on those specific health bar types (not to mention Maulers, Plasma Gunners and Twins heads aswell as Scab Ragers torso). Many people take Flak damage bonus but it is not needed; the weapon does very well againts that HP type as it is
- Hammerblow allows You to control both chaf and elites alike with ease; it is crucial for the weapon to fulfill its purpose as a single-handetly 
- Execution is an elaborate choice over High Voltage; stagger is universal and can be applied by anyone while electrocution status which High Voltage requires to apply its bonus damage comes only from Your own weapon and mines if You use them, Shock Mauls special attack, Smite and Electrokinetic Staff secondary attack... such a limited pool of sources for 5% more dmg IMO is not worth it
Another great choice is Shock Maul with same perks and Hammerblow and Skullcrusher blessings; mark does not matter; both are great so use what fits You better!
Other weapons are more or less worse options;
- Shields and Mauls offer great defence due to Hammerblow but their mobility is very poor when paired with the Boltgun
- Crusher is a great weapon both for offence and defence but its mobility is very poor when paired with the Boltgun
- Combat Axes are great weapons but againts big masses of armour they start to struggle; in this build it would be way worse to have them over even Shock Mauls IMO (not to mention some minor changes in the talent tree should be implemented to make them better)
- Tactical Axes offer very little stagger and may struggle againts huge masses of armour due to low cleave and reliance on crits and weakspots which this build is not build into
- Chainswords offer nice horde clear and acceptable single target damage but struggle againts carapace which Your Boltgun won't be able to deal with efficiently on its own thus i do not recommend using them
4. Ruthfless Efficiency aura is very important for the builds effectiveness; IMO its also the best Arby aura You could get teamwise... however due to talent tree structure its usualy not taken (most of the time You will have to sink in 1 extra talent point to get it in comparrison to Breaking Dissent or even Part of the Squad)
5. Forceful keystone is obviously a great choice but has a little surprise; Arbites Vigilant! While usualy used with Break The Line ability it shines the most in Castigator's Stance; when activated it gives your 70% damage reduction which offsets a downside of losing forceful stacks while buffing Your offensive capabilities and since You will be using high stagger weapons while its active You will regain them before the ability ends thus not losing Your main source of tankyness!
6. It is important to note a couple of things that will make using this build a better experience;
A) Mk III Boltgun has a mechanic where the explosion won't occur if the projectile hit a solid surface or an enemy within up to 3m distance from You! Keep that in mind and space Yourself well in combat!
B) Scavenge the map! Alot of the time people run past many spots where ammo may spawn in (that goes for havoc aswell!); this game ain't a race - better to take Your time than have less precious bullets that may just save Your run!
C) Use Your Boltgun often but do not overuse it! Micromanaging rezources is a very important skill in video games (in real life aswell ;p). Use it with Your ability and after it ended while the Writ of Execution buff is still active (if need be; utilize Arbites Grenade too!). In the meantime You are a classic frontliner with Your melee and grenades so get stuck in!
D) Castigator's Stance is barely ever used as a 'breacher' type of playstyle which IMO is a mistake; this build allows Your team to spearhead into the mass of the enemy and/or entrenched ranged units where You are the tip of said spear! Your destructive potential and survivability allows You to be more proactive and dictate the flow of Your teams attack!

Thats about it! I hope You will have fun playing this build and blasting away Your problems with ease!
Shotout to my friend Feisty Fiend (aka Serg) for the help with the image! (He gave the Captain a nice bath to clean the greenish hue off him XD)