Double-barrel / CAxe stealthy fragger
 Veteran Build - Warhammer 40k: Darktide 

This is a build somebody on game's official Discord's veteran class channel shared with me long ago, before the Bound by Duty update was a thing even. I've played it regularly since then up to Auric levels of difficulty, and had a lot of fun with it. So when BbD landed, I did my best migrating it to the new tree.

It haven't changed much since when I first tried it, only became even stronger now, in its melee part in particular. The general loop is still the same: chop a few mobs with caxe to get 3+ Weapons Specialist stacks, guaranteeing your next first shot with it will have 100% crit chance on the next weapon swap - and that one round will be loaded into your dbarrel when you'll swap to it; then swap weapons and shoot an elite or a bunch of chaffers with the gun. Blesses on the gun give that first shot an increased cleave and a guaranteed bleeding application, enhancing its deadliness, one-shoting foes like Mauler or any gunners, or mowing through a dozen groaners. Going through that loop constantly while fighting a hoard helps with keeping up +25% damage boost from Agile Engagement talent, thinning it down rather quick, saving you a few frags.

Use Infiltrate to safely close gaps to a bunch of priority targets (usually gunners) and drop a few frags while still in stealth (as underhand grenade throw doesn't reveal you, even after them exploding) - then drop stealth and start chopping, benefitting from +30% damage boost lasting for 5s even after revealing yourself.

Use Frag Grenades to clear the softer part of the hoard. Dropping two grenades one after another clears a room full of chaff in no time - while weakening "softer" elites. Depending on stacks you have at that time, it may kill gunners (except for plasma) in a decent range as well. Keep in mind that, due to how DoT damage is implemented in the game, it's paramount to minimize delay between the explosions, as it increases overall damage dealt significantly; so practice to throw the 2nd grenade after the 1st one as quick as possible (underhand throws are usually faster). As you have close to unlimited grenades supply here, dropping them every time when facing a dense enough hoard becomes mandatory, to utilize full potential of the build. 

A few notes:

• The build has an obvious loophole by not being able to effectively dispatch priority threats at range; even at medium range killing "softer" specialists becomes tricky and often requires you to shoot them twice; at longer distances it's even harder, thus you either will have to rely on your teammates to handle it, or close the gap on foot stealthed
• I prefer to have Overwatch sub-talent in all my Infiltrate veteran builds, as I feel it allows you to more fully utilize its potential. It's much more useful when you don't need to overthink whether or not it's okay to activate your Ability here and now for short-term gains, or save it for the later - with two charges you can do both
• Besides using Infiltrate offensively, you can use it defensively to insta-replenish your toughness and get +50% toughness dmg resist for 10s - as well as picking up and freeing your fallen allies (enhanced by +12% Ally Revive Speed perks on curios)
• Pushing foes with Infiltrate active doesn't interrupt it until no direct damage is inflicted, can be useful when making your way through a hoard. Pressing buttons or pulling levers, or picking up powerups don't drop your stealth either, and you can solve minigames, like hacking terminals, while stealthed as well. When picking up / rescuing allies, you are not dropping your stealth upon starting the interaction, but it will drop when it's completed
• Just one Weapons Specialist stack is required to reload one round into your gun, so if you need to shoot it urgently - one stack is all you need. Reciprocity talent also helps to close the gap to that sweet 100% crit margin, as it usually will give you extra +25% crit chance during a prolonged fight
• After BbD update it's now much easier to maintain Skirmisher talent's stacks, as it's now triggered by sliding; you may want to introduce dodge-sliding moves into your usual combat moveset, to keep up that sweet +25% dmg boost during a fight

Afterword

Infiltrate/Shroud builds have a bad rep for a reason. It's easy to get carried away with the freedoms they offer, as well as succumb to the convenience of avoiding fights at your comrades' expense. Thus, if you're still into that kind of playstyles, imo, the most non-toxic way to utilize them is to play them as any other combat-oriented build, just with a twist.

Stay with your team, don't stroll too far away from them, and mostly use the Ability for a couple seconds to heal/protect yourself and/or drop agro or get behind some foe's back, or drop frags/reload your gun - then unstealth and keep fighting with the group. Cases when it's really beneficial for you to run a long distance to activate something or remove some distant threat exist - but are much less common. With experience you'll figure out for yourself when it's really worth it.

That's it, have fun!