Shield of the Lex
 Arbites Build - Warhammer 40k: Darktide 

Of note

Terminus Warrant keystone seems tailored for the shotpistol. Ranged Justice grants cleave, so each shot hits multiple enemies. Helps with getting to Specialists hiding in/behind hordes, the shot goes right through. 

1 shot can hit dozens of enemies, so it's very easy to build stacks of both Ranged and Melee Justice. The extra reload speed helps out a ton as well.

Made Reload Speed the dumpstat, since there's barely any difference with all the reload buffs in this build, which grants me more mobility.

Full Bore and Run 'n Gun has huge synergy. Run 'n Gun tightens spread, making it easy to proc Full Bore. It also opens up opportunities since you can shoot without slowing down, which is really fun.

Target Priority makes a huge difference for both the maul and shotpistol, but it's  especially great for the shotpistol . 15% more damage to enemies above 75% health means you one-shot gunners when aiming center mass. 

Shock maul+shield does a lot of damage with all the buffs in the skilltree, so I went with Confident Strike to help keep my Toughness up instead of doubling down on damage blessings. Opportunist means you 2-3 shot crushers anyway.

Don't forget you have a shield both in melee and ranged. You can take your time aiming while blocking in 360 degrees, you can march up to gunners without a worry. 

Can switch out Break the Line for Nuncio-Aquila, no problem. Same with mines over grenades, if you prefer.

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Between the shields special attack(the flashbang), 5 regenning impact grenades/electro mines, and Break the Line you have a lot of ways to deal with stuff up close.  

Shotpistol becomes a 6-shot beast with Run 'n Gun, Full Bore, Priority Endowment and Terminus Warrant.