Arbites
5-20% Damage Bonus vs Staggered enemies.
10-25% Damage vs Electrocuted enemies.
6-12% Critical Chance for each Enemy Hit by your previous attack. Stacks 5 times.
Killing an enemy by firing both barrels makes your next Reload 40-70% faster.
Crucis Mk XI Double-Barrelled Shotgun + Priority Endowment and Blessed Armament means you can fire four shots before needing to reload when blasting into hordes, which as you might assume goes hard, though you want to avoid shooting TOO fast so that the 1.5 seconds doesnt elapse between the first of the two volleys per reload. Don’t use Castigator's Stance when firing into bosses as you won’t get the fourth round, just settle for the 3.
The keystone choice is there for more powerful bossing and anti-ogryn, as well as rewarding dumping shots into hordes of elites and specialists, plus additional crit to maximize damage on the quad barrel.
The Branx Mk III Arbites Shock Maul is there for armored enemy hordes (courtesy of shock mines giving free ups for the special attack in the case of crusher groups) as well as the powerful blessings and generalist abilites, but you can sub it out for the standard shock maul or the crusher if you want better boss damage instead.