Anti-Elite Support Bolter - Havoc 40
 Veteran Build - Warhammer 40k: Darktide 

Designed around hitting breakpoints on elites in high havocs. The build's major deviation from meta Focus Target! talent setups is investing in Kill Zone on the lower left side of the tree for more reliable breakpoints.

The combination of 6 stacks of Redirect Fire! , some Focus Target! and/or Pinning Fire stacks, Longshot , and Kill Zone end up hitting a lot of one-shot breakpoints on ranged elites and specials without having to rely on crits. Surgical combined with the above is necessary at the highest havoc levels for hitting desirable breakpoints on some outliers, namely Reapers and Flamers.

This build is inherently less versatile / has less solo-carry potential than dueling sword plasma meta builds, but provides exceptionally low TTK on priority targets and higher boss burst potential without sacrificing shout or focus fire. It fills a niche similar to Helbore builds, but with stronger defensive and burst characteristics.

Ranged Weapon

• Why Locke Mk IIb Spearhead Boltgun over Locke Mk III Spearhead Boltgun ? Ammo efficiency (more single-target damage per bullet and more ammo capacity), ability to burst bosses with weakspot hits in panics, and has cleave to kill snipers / bombers etc mixed into hordes (Mk III always detonates on the first target hit). The Mk III is superior for dealing with groups of regular shooters, but on balance I don't think it's worth it since if you have the rhythm of scope -> single-fire -> unscope before recoil fully registers -> re-scope you can get decent clearing capability without sacrificing the cleave needed to delete specials that can wipe a team.

• Why bolter over plasma? More ammo efficient if you hit headshots, lower TTK on particularly dangerous enemies like reapers, a little more flexibility in situations involving surprise armor or bosses, lower TTK for shooter removal (higher ROF lets you retarget faster), better movement characteristics, equivalent or better capacity for burst damage and stagger in panic situations (tapping m1 to shred or stagger a rager/bruiser/mauler clump or instagib a reaper can save you, special action push can push bursters and stagger ragers on weakspot)

  - Notable weaknesses vs plasma - reduced cleave / no wallbangs, cannot stagger muties/crushers/captains with headshots.

• Why bolter over Helbore? More capacity to quickly burst or solo bosses in emergencies, equivalent or better cleave, no reliance on stamina/crits for hitting breakpoints (can keep a health or third toughness curio instead of a stamina curio), better performance in panic situations

  - Weaknesses vs Helbore - much less ammo efficient, no huge instant brittleness team utility from Armourbane , no bayonet for captain shields, worse performance against crushers and maulers

Blessings

• Pinning Fire will almost always have at least 2 stacks and often the max 5 due to sheer density of enemies. Essential for breakpoints once specials and elites reach their maximum health bonuses from havoc modifiers (on levels 38 and 39 respectively). 
  • Note that stacks of Pinning Fire and Focus Target! each effectively add 5% damage , so you can calculate breakpoints with them interchangeably. E.g. shooting a bolter with 5 stacks of PF at an untagged enemy is equivalent to shooting with 3 stacks of PF at an elite tagged with 2 FT stacks.
• Surgical is essential for reliably hitting two-shot breakpoints against reapers on Havoc 39+ - at mid range you'll want two weakspots with at least one crit unless you have 4+ PF/FT stacks. Crits also give you a bit more room to bodyshot softer enemies like scab gunners and guarantee kills on flamers without needing favorable PF/FT stacks.
• Puncture helps with ammo efficiency / if you miss headshots on scab gunners while missing some kind of stacks on lower havocs; helps reduce cognitive overhead and retarget faster. 
  • Below Havoc 39 I prefer this over Surgical since it can help compensate for missing stacks without forcing you to wait for crit chance to tick up, but up to personal preference. 
  • Puncture might be viable at 39+ paired with Surgical in place of Pinning Fire if, but I think PF's flat strength/damage ends up being much more flexible / gives you some freedom to avoid relying on crits.
• Shattering Impact is generally not worth it since you're trying to one or two-shot your primary targets. Builds like Rumbler Ogryn and arc Skitarii apply brittleness much more effectively. Kraks are generally the better option for armor packs, and your melee should be able to handle isolated armor. For unyielding bosses you should be leaning on kraks + max focus target stacks. Power or Dueling swords can help finish them quickly.

Perks 

• +Unyielding is effectively required for hitting two-shot breakpoints on Reapers on Havoc 39+ - without the perk you need two crit weakspots and some PF/FT/Longshot stacks.
• +Elites and +Flak is a tossup. 
  • +Elites helps with more reliable reaper two-shots on H39+ - with 4 FT/PF stacks, two non-crit weakspots is enough. Extra damage on plasma gunner headshots is also welcome (flak bodyshots are lower ttk, but sometimes you can only see their heads over hordes)
  • +Flak helps killing scab gunners without needing crits. With +Elites, you need ~5 total stacks of FT/PF to kill on H39+ with a non-crit weakspot. 
  • I prefer +Elites because scab gunners are usually positioned in groups, so it's easier to build PF stacks by shooting at them and facilitate easier one-shots in the event you miss one. Reapers won't get staggered by bolter headshots, and shooting up at their heads doesn't stagger as many nearby enemies as shooting into a horde, so you won't get as much free damage on subsequent shots.
• Sub H39, +Unyielding doesn't change breakpoints on reapers much - with or without, they're two-shot headshots (or one headshot one bodyshot with some PF/FT stacks). +Flak and +Elite works fine there.
• If team lacks answers for quickly deleting twins, can take +Carapace to allow for one-phasing Rinda. Otherwise not as useful as other choices - helps if you need to panic clear some crushers or maulers, but kraks are generally better suited for armor packs if you have them. With plasma gunner and mauler headshots you'll get equivalent TTK by hitting flak bodyshots and for scab ragers you should always be going for headshots

Blitz and Talents

• Taking Krak Grenade enhanced with Grenade Tinkerer lets you preserve your bolter ammo for elites/specials. Use grenades liberally against armor packs and bosses. Smokes and frags are both excellent, though! Choose what works best with your team composition.
• Very few talents can get moved around here if you want to keep the necessary 6 redirect fire stacks, Kill Zone, and Superiority Complex. 
• Volley Adept is basically required for Bolter - it's the single biggest reload buff you can get and you'll have 95%+ uptime. Further reload nodes or a +reload perk on the bolter aren't worth it imo. Just practice cancelling the reload animation right when the final sound cue starts when pulling back the slide (to see exactly when, watch when your ammo counter changes from 0 to 15) - cancelling with a sprint is fastest, but the push special action is only a tiny bit slower than a sprint cancel and you can do it while dodging. You can buffer ADS out of special action, too.
• One Motion is theoretically optional if your awareness is cracked enough, but in practice it's very helpful since you'll want to have your melee weapon out for mobility most of the time, and that accelerated draw time will absolutely help you catch snipers and bombers. You can always shout to cancel the draw animation, but it's best to keep that capability for emergencies imo since instant gold health + kb + revive is a critical team resource.
• Only In Death Does Duty End is also theoretically optional. In practice, revive shout utility seems like too much to pass up relative to any single other node, especially given the build's lack of staying power or AoE CC options aside from shout.
• If you elect to pass on one of the above, decent candidates for replacement are: the 15% melee damage boost node just above Close Quarters Killzone (helps with poking twins in the head and makes horde control more manageable), Exhilarating Takedown (for better sustain and tankiness - expect 45-60%+ uptime), or Born Leader for enhanced shouts

Melee Weapon

This build lacks nodes enhancing melee damage, so there are compromises here.

Achlys Mk VI Power Sword with Slaughterer is the best way to kill chaff while keeping capacity to chunk armor and bosses. +Carapace will let you kill crushers and maulers quickly without spending resources, and will help you do meaningful damage to twins. +Unyielding is also helpful for bosses and bulwarks in particular. 

However, the combination or mediocre runspeed and dodges, and vulnerability during activation window (mitigated only by freezing stamina regen for sliding activations) make you less capable of clutching rescues or revives when shout is on cooldown.

Other options depending on team comp and personal preference:

• If team lacks a source of Skullcrusher  and/or very high single-hit damage to skip captain phases, you can take a Shock Maul. The special attack will chunk captain shields and make their fights go quicker. I prefer the Munitorum Mk III Shock Maul.
  • Skullcrusher and +Maniac is non-negotiable (dreg ragers are your nemesis and muties take a couple of weakspot hits to kill)
  • I generally prefer Hammerblow and +Unarmored for safety and horde clear (+Unarmored helps with groaners, traitor moebian and scab bruiser weakspots. +flak doesn't seem to hit valuable breakpoints e.g. dreg bruisers). Munitorum Mk III Shock Maul H1 -> L1 loop will keep you safe and prevent you from getting tripped by unkilled chaff
  • Alternative setup with more ability to kill armor in a pinch - High Voltage and +Carapace. Doesn't kill/ knock back chaff as well though
  • Extra dodge and block efficiency relative to powersword helps with survivability, but maintains mediocre runspeed
  • Stunning crushers and unshielded captains with special action is extremely funny but requires forethought and usually isn't worth it
• If you struggle to survive with powersword and want to prioritize mobility / staying alive and to better stall for shout revives, take a pokey dueling sword  - can't go wrong with Maccabian Mk IV Duelling Sword 
  • Suggest Thrust (for poking muties) and Uncanny Strike  (you'll want the on-demand brittleness for armor). Without melee damage or crit nodes you won't be getting much benefit out of Riposte or Shred . Agile is neat but dueling sword already has plenty of dodges, and extra melee armor damage is very much welcome on this build
  • +Carapace and +Unyielding for efficient armor removal and boss contribution
  • Dump cleave damage for mobility
  • Rely on your movement attributes and parry to avoid getting bogged down. Getting isolated and bodyblocked by chaff is your biggest threat.

I generally find power falchions underwhelming compared to the power sword without extra damage nodes, and they lack the utility of shock maul, but they are also reasonable options. Tac Axes are a decent dueling sword sidegrade as well - being able to interrupt overheads and charging muties on special action weakspots is nice - but it'll struggle even more with armor and can't contribute as well for bosses.

Synergies

• Works very well with anything that applies mass brittleness - e.g. Lorenz Mk VI Rumbler Ogryn with Shattering Impact , arc Skitarii with System Shock 
• Has exceptional synergy with other veterans - the Fire Team aura helps hit breakpoints even more comfortably / gives you more room to bodyshot. Plasma's biggest advantage relative to bolter is its increased cleave / greater reliability killing enemies in cover or behind walls, so having a plasma vet can plug holes in bolter's coverage. Taking kraks also frees the other vet to take smokes or frags which each have great team utility.
• Horde-control specialists - e.g. Rifthaven Mk II Inferno Force Staff or Equinox Mk III Voidblast Force Staff or greatsword psykers, taunt ogryn, relic blade zealot etc - help keep space open for you to focus on distant threats.
• Works well with the Dog Whistle mod. I suggest auto-targeting: bombers, snipers, dreg/scab/plasma gunners, reapers, and Rinda Karnak / melee twin (tracking her quickly is hugely helpful). Flamers and trappers are okay to include but are usually easier to spot due to their aggressive movement. Set auto target cooldown minimum to 2 or 3 seconds to balance having healthy focus fire stacks and quickly identifying priority targets (setting the restriction by focus target stacks instead of by time exhibits some inconsistent behavior, esp when there are multiple focus target vets on a team). I also recommend having an easily accessible toggle bind for when you want to ensure you keep 6 stacks on a boss (I use the grave/tilde key).

Similar Builds

• [META] NRGaa's Bolter Veteran Havoc 40: Very similar concept, but sacrifices Focus Target team utility and some survivability in exchange for more even more consistent breakpoints and better melee performance. A good read - the positioning, blitz usage, and enemy prioritization guidance there applies to this build.
•  Two Tapper by ripthejack3r: Similar concept and almost identical talents, but uses the Mk III and has different outlooks wrt weapon setups.

Bonus Variation: Skirmisher

Has a higher damage ceiling than the prime build and is much more effective in melee, therefore more flexible. It's squishier, however, and needs to sacrifice a valuable utility node relative to the prime. The most likely candidates in order of salience are One Motion , Grenade Tinkerer , Iron Will , Only In Death Does Duty End, Fully Loaded (discouraged since this will also lower ammo pickup counts)

Talents: Remove Kill Zone and the preceding white TDR node, Focused Fire , and one of the aforementioned nodes. Substitute with the white 15% melee damage node, Reciprocity (or Close Quarters Killzone ), Desperado and Skirmisher . If removing grenade tinkerer, suggest swapping blitz.

Bolter: No major change in guidance. Can probably take +Flak instead of +Elite.

Melee: PS is still excellent, but there are many more viable options. I favor Maccabian Mk IV Duelling Sword with Thrust and Shred , or Atrox Mk IV Tactical Axe with Headtaker and Shred 

Bonus Variation: Deadshot Critseeker / Helbore Cosplay

Less good than the prime build and imo essentially a goofy and suboptimal shout helbore build, but good fun.

Curios: Swap +health curio for +3 stamina curio. Swap +5% toughness on stam to +5% health, swap corruption resist on one of the other curios for another +5% health perk (together, just enough to not get 1shot by a groaner).

Talents: Swap One Motion and Grenade Tinkerer for Deadshot and Target Down! . Suggest swapping blitz to smoke or frags.

Bolter: Unchanged

Melee: Unchanged