The sentinel CANNOT build it's own stacks of [Galvanized Chamber] or Aptitude.

You need to carry a RIFLE also with [Galvanized Aptitude]+Chamber equipped and get kills to build stacks for the sentinel.

If you are NOT using a rifle with those mods, then just swap galvanized mods out for [Serration] and [Split Chamber].

[Thermite Rounds] flexible for Primed Faction mod to min max damage. [Vigilante Armaments] is an alternative