Autocannon cosplaying as an Autogun
 Veteran Build - Warhammer 40k: Darktide 

Build Core: 

1. Shoot them in the head. This build is focused on shooting, a lot, and is very weakspot oriented so don't miss or you'll be hurting on ammo. Displayed build has been tested on Havoc 1-29 without Rotten Armour ; I have not tried it in high Havoc but I can't imagine you'll have an amazing time blazing away at Carapace with such low ammo pickups.
2. 25% Carapace is required to make heavy armor hits worth the ammo consumed and 10% Elite Damage adds even more on top of that while also helping with everything else. 25% Flak did not change TTKs or breakpoints enough to matter
3. The 25% rending from Rending Strikes and Chink in their Armour is required for legitimately good and efficient armor killing with the Agripinaa Mk I Infantry Autogun ; can hit 4000dps with 15 Marksman's Focus stacks with no crits. Don't bother shooting heavy armor until you hit at least 10 Marksman's Focus stacks for Chink in their Armour's rending.
4.  Punishing Salvo procs help ammo efficiency; if you can get the rhythm down, you'll save a lot of ammo. If you're not consistent with headshots, can sub Punishing Salvo for Sustained Fire. I found that Fire Frenzy and Deathspitter are too reliant on making recent kills, so despite the overall greater potential damage, I do not recommend it since you're intended to use this build for anti-armor along with general anti-elite/special. Further, since we aren't bringing any reload talents, one reload wipes out all stacks.
   1.  After much testing, I found that learning how to use Punishing Salvo/Sustained Fire ends up having similar TTKs for armor groups with similar or better ammo efficiency as starting with max stacks of Fire Frenzy or Deathspitter.
   2. This entire build is based around the Agripinaa Mk I Infantry Autogun ; if you use a different mark Autogun, SusFire and PunSal is much weaker and thus ammo efficiency is much worse. Change it at your peril!
5. The Bigger they Are ... is mandatory for maximum Executioner's Stance uptime and Reaper highlighting/refreshing.
6. One +HP Curio to avoid poxburster one shots on higher Havocs

Personal Preference:

1. Melee weapon: completely up to you, but the Power Falchions have the best mix of mobility, horde clear, and armor damage but if your front line is amazing and you have good comms to assist when you get flanked by huge armor packs, the Achlys Mk VI Power Sword is of course, the best. If you are going to use your melee weapon as a repositioning tool only (not recommended), drop Trench Fighter Drill for something else. Or if you want to be an ammo hog.
2. Blitz: Shredders for breathing room; Kraks if you're a movement beast and want to save ammo against clustered armor. You won't need Smokes on an individual level, but if you want to support your team, hell yeah
3. Curio perks: No ability regen required on curios unless you want to spam ult, even when it's still active, for Marksman's +20% Strength on weakspot. Gunner Damage Resistance isn't as mandatory as it normally would be since Toughness regen is pretty high with ExeStance when combined with Exhilarating Takedown and often Confirmed Kill 
4. Marksman: This is a great talent when using it to maximize ammo efficiency against armor but if you feel like you would rather throw another grenade, feel free to sub it for Grenadier, especially in Rotten Armour or just High Havocs in general
5. Confirmed Kill: If skilled enough, you can drop this on Auric difficulty for any other node you prefer since The Emperor's Fading light (I)/(II) isn't active. When on Havoc, you'll want it for safety but I'm not exactly the best vet player ever so if you're confident, go with grace.
6. 10% Elite Damage can be subbed for 25% Unyielding for more boss damage and potentially one or two rounds less on Reapers. I personally really like having even more carapace damage and as stated earlier, 10% Elite Damage helps with every other non-Reaper special/elite so I found this to be a strong perk to keep on the gun