Basic arby advice
 Arbites Build - Warhammer 40k: Darktide 

You have one extra point you can pretty much drop anywhere. Extra ammo. zealot dedication. imposing force. The emperor's fist. Really depends. Most of this is based on tanking and just being hard to kill front line. Abuse your ultimate. Build forceful quickly. Hold the line. 
You can also switch man and cyber mastiff with walk it off or shield plates depending on preference. 

Arbitrator shotgun, 6 round variant, is the best. However that's for serious havoc gameplay. In random aurics I recommend the 13 shot shotgun. It builds forceful and when you ads you can engage targets super far, like a sniper across the map. This added versatility makes random games much easier. Also if you run expedition it's basically mandatory. If you don't run Arbites Shock Maul and go for shield then you might want to drop true grit and swap the health curio to either stamina or toughness. Because shield, as long as you have stamina, literally blocks a crusher overhead or sniper shot, but have it at 80 defense and dump power output. True grit then becomes nearly useless only activating if you are just caught asleep. Also if you do pick maul and shield then might want to switch man and cyber mastiff with shield plates for obvious reasons. This case you have two extra points. For Arbites Shock Maul you can always go with extra staggered and electrocuted damage. This plays nicely with your dog electrocuting enemies which we took. For shotgun plenty of good stuff. Honestly just pick and choose.