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.

Shovel - great offensive and defensive option. Duelling sword is obviously OP, and Powersword is another good alt.
Perks:

• Elite - Shovel special WP hits will stop mutants, so you don't need extra damage vs them. Maniac is fine alternative.
• Flak - to help chew through all the Scab ranged and melee units.

Blessings

• Decimator  - Since we are running Deadshot we are often low on stam anyway, you can run All or Nothing instead.
• Uncanny- Grants good (multiplicative) damage boost vs all armor types, and greatly increases Carapace damage.

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