Infinite Ammo Ghost Laspistol (now with video!)
 Veteran Build - Warhammer 40k: Darktide 

1. Proc Ghost, which will proc Desperado (20% crit) and Reciprocity (25% crit) along with refilling your stamina through Duck and Dive.
2. Aim, activating Deadshot (25% crit)
3. Enjoy your near 100% crit chance, and infinite ammo from Shock Trooper.

Melee is your choice, I like a catachan sword for extra melee safety but chainaxe is good for rotten armor. You one shot shotgunners and most specials, and your crits do high damage to flak and not too terrible damage to carapace. Krak Grenade is nice to have but not necessary, same with Executioner's Stance (I just like it for the wallhacks and toughness regen).

Alternate build could be Infiltrate, swap Ghost or Desperado for Raking Fire, and take Covert Operative instead of whatever talent you deem unnecessary.

I've recently started using knife for even more mobility and got a pretty good havoc game with my premade. Ghost really shines in big skirmishes, you can just sit there and keep plinking at gunners as long as you're accurate. The video linked is only a havoc 32 but I've seen good success with it in 40s also. In the video I've switched to Voice of Command for a more supportive setup, albeit with a bit less accuracy and damage. It still performs great though, as long as you're able to catch dodges outside of Telekine Dome.