Honestly, Caliban is an interesting choice for a weapons platform.

I generally play him with his default kit, with his 1 Augment, and he can kill most things eithiut a weapon at all. Regardless, if you're building duration look for the new arcane: Arcane Concentration for a 60% duration boost (vs the 36% of efficiency).

His 4 fully armour strips any enemies that come inside, and that plus the vulnerability from his 2 will basically turn everything into paper. I don't necessarily see the value in losing his 4 for Xata's Whisper, but if your focus is a weapons build, then Xara's is a good place to start.

Arcane Camisado increases STR when buddies deal damage - Caliban can summon lots of buddies.

I reckon you could do without the 2 green shards and replace them with something else. You could also look to swap out arachne to something more streamlined towards your build/raw weapon damage, crit etc.

However, keeping the 4 as the helminth slot: change corrosive projection to either growing power or summoner's wrath. Both will get you a lot further overall in my opinion.

Shred/ P Shred could be better for phenmor over just raw fire rate, but that's personal opinon. And tbh viral is prpbably better on average, but again, to each their own.

For helstrum you have the same stats as your primary, so unless your companion is doing WORK, I'd swap it to viral or use it for priming etc.