Helbore overwatch ( Havoc )
 Veteran Build - Warhammer 40k: Darktide 

Build Alternatives :

1. If you have a harder time creating space and position yourself properly, you do not have good horde clear on your team, or your team possess extremely high armor and boss damage, then swap Krak Grenade with Shredder Frag Grenade.
2. If you want to rely less on grenades, swap Demolition Team and Grenade Tinkerer with Rending Strikes + ( Chink in their Armour or Bring it Down! ). You will have less blitz and they will be of lesser use, but each shot of your Lucius Mk IV Helbore Lasgun will be greatly increased, allowing you to even swap flak armor damage to unyielding damage.
3. You could also swap out Demolition Team alone for Toughness Damage Reduction ( 10% ) if you desire to be tankier.
4. Both marks of the Munitorum Mk III Shock Maul are great here, both being used to stagger bulwark and crushers when necessary, bringing down captain shield with ease and give you enough stagger against normal enemies to survive if needed. the only change being your attack pattern.

Strength :

• You have an amazing range efficiency. From small gunners that will die in one non-charged weakspot shot, to Reaper  that will be one shoted with a charged weakspot shot once you have 10 stacks of Marksman's Focus, everything in between being annihilated by charged weakspot shot. Weight of Fire allowing you to chain again and again with quicker charging, while Surgical will give you crit chance to not consume ammo all the time ( you do not need to have 10 stacks to fire, it charge quickly no matter what and even 30% is enough :) )
• You also have really good armor managment, between Krak Grenade for Crusher and Bulwark, or simply chaining headshot against anything armored, you will support your team heavily. ( 3 crit headshot on crushers to kill them. doesn't account for Rotten armor for which you need to rely more on krak grenades for large groups and Munitorum Mk III Shock Maul special for Crusherstagger. )
• While you are Killing any enemy that come your way, your immunity to suppresion from Veteran natural passive paired with an insane Resistence with Confirmed Kill, Exhilarating Takedown, Close Order Drill, Iron Will, Catch a Breath and Voice of Command will allow you to mantain a very high ammount of toughness.
• As you are using Born Leader, Confirmed Kill + Catch a Breath + Exhilarating Takedown + Voice of Command will regenerate an insane ammount of toughness for your entire team constantly on top of providing Golden Toughness. 
• Considering you actively stay on your range weapon chaising range one shots, only swapping to melee when absolutely needed, Tactical Awarenesswill be active a lot more than usual. Additionally, your 3 curios have Combat ability regeneration, making your Voice of Command come back even faster.
• Taking into consideration the fact that your team survivability is directly tied to your own ( not being able to clear hordes and needing some space to opperate ), Only In Death Does Duty End is a cornerstone of this build, making you able to get out of otherwise impossible situation to handle on your own.
• Between Fully Loaded, Shock Trooper, Survivalist, Surgical and the raw ammount of ammo you posses, your ammo economy is a amazing, even while basically only and always using your Lucius Mk IV Helbore Lasgun, allowing you to give more ammo pickups to your team which overall augment your range density.
• Krak Grenade with Grenade Tinkerer paired with constant headshot give you extremely reliable and potentially fast ( if you use a lot of grenades ) kill on monstrosities.
• The special attack of Munitorum Mk III Shock Maul will allow you to disable Scab Captain shields quickly, letting your team damage them as you switch focus to other range targets.

Weakness :

• You have actively horrendous horde clear. the build focusing on elite targeting and team support, your entire " horde clear " is your team. If your team is dead, your horde clear is dead.
• Even so you can break Scab Captain shields easily, you do not have great melee damage against them afterwards, making you slow to deal with them on your own if you are ever in need too.
• You need space to play this build properly, and even with a team that actively create it for you ( dealing with hordes mostly ), you need an extremely good understanding of where you need to position yourself. You aren't " reacting " to enemies, you're " thinking " of where you need to be based of where the enemy will come from at all times. This ask for experience, understanding of spawns places and safe spots, mastery over your weapon damage + swap speed and priority. 
• If you ever run out of Krak grenades during raw armor spamm, even if you still have options with repositioning, weakspot shots and Munitorum Mk III Shock Maul stagger, you will feel slow against to unrelating assault that armor spamm can be.

Conclusion :

You are a team player, you need your team alive, you keep your team alive and your give to your team amazing constant range support with low ammo cost. You're still a veteran, which makes the grenade economy of your team that much better. 

It is a hard build to play. Much less reactive, much more proactive. Requiring much more experience, havoc understanding, and build mastery than most out there. Your positioning need to be flawless, you need precision in your aim ( this weapon start to be amazing only when you hit headshots ), it'll ask you to be slow and methodical in everything you do ( you cannot unga bunga up to any enemies. )
But, BUT ! 
It is rewarding once you check all the needs ;) it is extremely fun ! You feel like a true veteran picking priority target left right and center while dictating the pace of the game, the health of your team and it's economy. 
Understand where you need to be ! make your team follow your lead ! and destroy the enemy ! 

Obviously not meta,  but still extremely strong, made to be used in the highest level of havoc !
( please, get a psyker on your team, if not a very heavy horde clear / stagger team around you )

I encourage you to try it out and tell me what you think ! ;)