Macharia
 Veteran Build - Warhammer 40k: Darktide 

This build has been used with success on everything up to Auric Maelstrom, serving as a general purpose build that can handle any enemy type without too much difficulty and fill whatever gaps a team has.

For range, I've chosen the Mk VII Headhunter/Vigilant autogun for its long range and precision, avoiding the team ever being stuck with only short ranged guns.
Between damage talents and opening salvo, the Mk VII can one shot many classes on a headshot (all light enemies, Gunners, Shotgunners, Bombers and Snipers), and between Ghost and toughness talents you have near invulnerability to incoming fire for as long you can chain headshots. (And even during reloads you can have a lot of toughness damage reduction built up). The dump stat of choice is Mobility in order to maximise ammo, damage, stopping power and stability. (80% ammo is necessary to get 23 rounds in a magazine).

The choice of an Infested damage perk may seem odd, but combined with Opening Salvo (which resets extremely quickly) it puts the weapon past a damage breakpoint where it can kill groaners and poxwalkers with a body shot; with enough cleave to kill two per shot, it actually becomes a surprisingly effective horde clear gun (at any range).
Elite damage is necessary to be able to headshot dreg gunners at any range (without it, damage falloff means they'll survive at longer ranges, even with maxed Damage and Stopping Power stats).

While Survivalist and Executioner's Stance would theoretically be great for this role, they require a number of other picks I'm underwhelmed by (I don't rate critical hits unless they're triggering a bonus other than just damage, and anything that relies on having no enemies nearby is either of limited use or requires being out of coherency with the rest of the team). I have instead taken the Fire Team/Voice of Command route which, aside from being good themselves, allow efficiently unlocking a large number of useful talents (Krak grenades and a number of useful buffs for them).

It's also a simple reality that it's not viable to run a pure marksman build without a very cooperative and coordinated team, so a lot of the time you'll find yourself in melee. At the moment, I'm running a power sword for that. Thrust is perhaps not the most popular pick for the power sword, but it means a charged heavy attack to the head can one-shot anything except an Ogryn, Monstrosity or Mauler. I ran Slaughterer for a while, but given the Vigilant Autogun isn't effective against Ragers, something that was something of a must have.

Power Cycler is, of course, a must-have pick.

And for the large portion of time you're stuck in melee, I've stacked additional toughness regen on kills and melee damage to boost this up.

Given this means I'm already stacking a lot of toughness regen, I've opted to go through Born Leader to get down to the popular Tactical Awareness and Iron Will picks; popular though it is, I don't hugely rate For the Emperor, as it's only got a short window of effectiveness, and Voice of Command already knocks back the enemies you'd be trying to kill anyway, making that even shorter.

As much as Born Leader might not be a hugely popular pick, with the amount of toughness regen this build is running, having that there as a constant passive has its merits (particularly with the additional aura radius from Charismatic).