VetBoltgun
 Veteran Build - Warhammer 40k: Darktide 

With the finesse buffs, Boltgun is just a sniper cannon in disguise. One shot body shot almost everything if you crit, head shots and ult for Ogryn, and melee elites.

Heavy Sword - And excellent horde clearing option even in Havoc, it can even deal with mixed hordes if there isn't carapace in it. Since we have Boltgun for Armor, this weapon makes a lot of sense. Cleave targets is normally the dump stat as we are running DeathBlow
Perks:

• Unarmored - this is a free slot, unarmored will help on dreg ranged elites, and scab bruisers when they have additional toughness from modifiers. Can easily be swapped with Elite or Unyeilding. Consider maniac d
• Flak - to help chew through all the Scab ranged and melee units and some elite enemies

Blessings

• DeathBlow - Heavy attacks will head shot many units in the game even in havoc 40, allowing this to have great value.
• Headtaker - Massive damage boost that is basically always on - hard to say no.

Boltgun
Perks:

• Unyielding- if you have a good anti-boss melee weapon take Stamina or Carapace instead
• Maniac - Helps to body shot flamers without Exe Stance, and one shot headshot mutants during ult. can be replaced with Unyielding\Reload if you want more boss damage depending on you melee weapon and team.

Blessings:

• Pinning Fire: 25% damage boost that is almost always up while firing - helps to one shot body shot ranged elites/specials depending on perks. Shattering Impact is only worth it for increased carapace damage, and even then crit headshots will decimate Crushers even without this.
• Surgical: With buffs to finesse, Critical body shots will now one shot all ranged Elites, and ranged specials making this a very strong blessing.
   

Make sure to use your Exe Stance to equip your Boltgun, as it will bypass the equip animation you normally have to go through if you normally swap to it. 
Target Down! with some extra Stam from Curios and or Weapon Perks will allow you to keep Deadshot up with very little cost so long as you are killing elites and specials. 

Currently Kill Zone and Opening Salvo I view as pretty similar - the former allows for a higher ceiling of damage, the latter is more consistent damage.

When fighting Monster use shred grenade into nearby enemies to charge Pinning Fire, and then you can just dump into the boss.
 
You can replace Exe Stance with Shout by getting rid of Exhil takedown or Opening Salvo
You need Volley Adept to be able to reload and not drop Exe Stance

You can replace Exhil Takedown  or Opening Salvo for Redirect Fire!  as that will allow you to kill Dreg Ragers without the Maniac perk and will just generally boost damage very consistently.