Bolt'Fun ( Havoc 40 )
 Veteran Build - Warhammer 40k: Darktide 

Build Alternatives :

1. The Locke Mk III Spearhead Boltgun can be manipulated in many different ways. You can keep the Mk III and change one of the 2 blessings for Surgical ( Preferably Puncture if you do ). You could swap for the Locke Mk IIb Spearhead Boltgun for better Armor damage and close range fighting, potentially changing blessings in the process ( Cavalcade, Fire Frenzy and Pinning Fire becoming much better for this Mark but having worse ammo economy and overall pinpoint accuracy ). Finally, you could also swap out Flak armor damage for Unyielding damage if you use it more against bosses ( it will remove your ability to one shot + Puncture some Flack armor elites, making your ammo economy worse )
2.  The Aridin Mk I Power Falchion Also have a few Alternatives. Even so i wouldn't advise changing the Mark, this weapon possess quite a few strong and interchangeable blessings. Wrath being the Blessing you would change in this case, here are your possibilities : Decapitator ( a strong option to stack more overhead damage but will be harder to stack than other blessing due to the condition of one shoting an enemy for it. ); Energy Leakage ( for better gradual damage against any targets ); Heatsink ( Allowing you to stay in melee longer by letting your special stay for longer ); Overload ( Creating a pocket space on lockout which can be very useful to swap to your range option and start doing your Vet work ); Counterattack ( This weapon special being fast to activate and deactivate contraty to it's sibling Munitorum Mk II Relic Blade, making the block and attack speed following the block much easier and consistent to get ). Keep In mind that even so all of those options are strong, Wrath will make you cleave trough hordes with ease, which can be needed quite often.
3. If you desire more offensive options, you can replace Out for Blood with : Grenadier ( More kraks can NEVER be bad ); Skirmisher ( 25% damage at max stack will definitly add up to your playstile ); Marksman ( Even so you're using Voice of Command which have longer CD, 10s worth of headshot damage will make a huge difference ); Withering Fire ( Interistingly enough, it affect Puncture which should only be on the first hit, but now is on all direct hits AND some enemy on the explosion range. )
4. If you desire to play with Only In Death Does Duty End, you can sacrifice Out for Blood or Desperado.
5. You could replace Krak Grenade with Shredder Frag Grenadeif you desire more horde clear at the cost of Armor and Bosses damage. Outside of Rotten Armour you still have strong options against Armor with both your melee and range weapon, but you will see a significent change in your monstrosity damage, even more so if you do not take Unyielding damage on the Locke Mk III Spearhead Boltgun and Puncture ( Do not swap it out if your team doesn't have Boss damage in the first place ).

Strength :

• You can do Everything well. Getting surrounded by a horde ? your Aridin Mk I Power Falchion special melee paired with Voice of Command will give you space to fight, survive and swap when needed. A monstrosity comming ? Nothing a few Krak Grenade, a few weakspot hit from your Aridin Mk I Power Falchion or your Locke Mk III Spearhead Boltgun, and stagger from Voice of Command cannot handle. Range enemy spam ? The Locke Mk III Spearhead Boltgun will not only Kill them easily, but will also stagger them and stop them from shooting on a large radius. A Scab Captain or Twins rushing you ? Focus Target! will make it much easier to deal with. Crusher and Scab Mauler hordes ? Krak Grenade, Voice of Command stagger and Aridin Mk I Power Falchion Weakspot hit will deal with it for you.
• You can support your team in any situation, and allow you to swap your position on a moment to moment basis. If you team need range, back out of a fight and start using your Locke Mk III Spearhead Boltgun . If your team needs melee support, go into the fray and create space with Voice of Command and Aridin Mk I Power Falchion.
• Demolition Stockpile and Demolition Team will keep your Grenade economy and allow your teammates to use grenade themself as you do not need to pick up any.
• Focus Target! Will not only allow you to direct the fight and bring down harder targets easier, but it'll also give your team a significent damage boost with Redirect Fire!.
• Locke Mk III Spearhead Boltgun is polyvalent, allowing you to kill Crusher and Scab Mauler with your range option.

Weakness :

• The Locke Mk III Spearhead Boltgun not being a Las weapon, and even while being supported by Fully Loaded and Survivalist, have 117 shots total. Most enemies will die in 2 shots without headshots, and only some will die in headshot + Puncture, basically meaning that you have around 80 shot until empty on average. This Ammo usage and economy make you have to consistently pick up ammo, significently reducing the ammo economy of your team.
• Even while having good and consistent boss damage in your build, you do not have any good way to deal with Scab Captain shields, making Rinda Karnak rush hard to deal with if you are the one she is focusing.
• The Locke Mk III Spearhead Boltgun require specific timing to take out considering his slow swap speed, while your Aridin Mk I Power Falchion need his power activated to do any damage at all. This will ask you to be much more precise on where and when you swap your weapons. ( One Motion doesn't change this in any meaningfull way )

Special Info :

Both the Aridin Mk I Power Falchion and Locke Mk III Spearhead Boltgun have specificities in there usage, that i will help you with below.

Aridin Mk I Power Falchion work similarly to the Munitorum Mk X Relic Blade :

• You can chain Heavy overhead without going back to heavy laterals by ether doing a quick attack after 2 heavy overheads or a very fast block animation canceled back into overheads. That move set of 2 heavy overhead followed by 1 light weakspot stab is your main way of dealing with any armored enemies and bosses. ( Even more so when they are pinged with Focus Target! )
• Lateral Heavies aren't as good as light attack spamms for horde clear.
• Letting your weapon overheat without having Overload blessing equiped is terrible, it'll be much slower to get back your special ( the 25% reduction from Overload being exactly the time increase without on lockout ).
• Your special activation and deactivation also count as a block ! ( you won't block a Crusher or Scab Mauler overhead ! ) which can be used to ether block in tighter situation or activate Counterattack if blessed with. The window of block is much greater and smooth than the Munitorum Mk X Relic Blade allowing you to do it more consistently if you bless into it.

The Locke Mk III Spearhead Boltgun benefits from quick swapping tricks.

• Executioner's Stance on Veteran will swap to the weapon almost instantly.
• Voice of Command while swapping on Veteran can make the swap smoother and quicker.
• Blades of Faith on Zealot will make the swap almost instant just like it would for the Artemia Mk III Purgation Flamer.
• You can also push Poxburster with the special hit of the Locke Mk III Spearhead Boltgun !

Conclusion :

As you might have deduced from the lower ammount of Weaknesses and Strengths than usual, this build is a do it all, master of none. You are very flexible in your positioning and can go from frontline to backline easily. You can direct your team towards specific targets and make them have an easier time in tougher situation.

But, BUT ! Finally a build that use the Locke Mk III Spearhead Boltgun on Veteran that works in any kind of scenario !

Definitly not meta ( again ahaha ), will punch lower in term of damage than the typical Grenade Veteran or Plasma Gun Veteran, but will do it's job, and will do it greatly even in the highest level of Havoc. 
Fun, Very fun in fact if you miss the good old days of Bolters being Bolters and not excuse for Grenade area. 

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