Disclaimer: This gets kinda ramble-y so I'll attempt to trim down the fat over time

I'm building specifcally for double pistols since they look cool. I doubt they are practical by end game standards but I will see if I can find a way to make them work. Currently [Akvasto] seems to be the best option but I do love the way [Aklex Prime] sounds. Perhaps switch between them based on the enemy type.

This is build is also aimed at solo play since thats what I have 400% Ability Strength. I guess it would be optimal if you switch between [Corrosive Projection] and [Shield Disruption].

Summary of the Build

PASSIVE
Overshield capacity doubled.
Start missions at maximum energy.

Pillage synergizes with Penance and Covenant. You use Penance then Pillage and Covenant to take out everyone. Use Thurible to regain energy. Rinse repeat.

Build details

Pillage costs 50 energy by default so I'm guessing it costs 48 with this build since Penance costs the same originally.

I picked [Corrosive Projection] so I can strip all Armor or Shields in one cast with less then 400% Ability Strength.

This specific variation uses Sling Strength from the Maudrai Focus Tree so every 20 seconds you need to Void Sling twice to get the 40% Strength. I choose this over Molt Augmented or Molt [Vigor] since I think its easier to proc in the beginning and Molt [Vigor] is a one time use so you have to keep using an Operator Ability between Pillage casts which sounds like a hassle.

Anyway heres how I get to 400% Ability Strength:

40% from Sling Strength
30% from 2 Crimson Tauforged Shards
77% from Umbral Intensify
55% from Transient Fortitude
99% from Blind Rage
Total = 301%

You start with 100% Ability Strength by default so: 301 + 100 = 401

Thurible Energy Per Kill Formula:
Energy Per Kill = 1 + (Energy Channeled × 15% ÷ (2 - Ability Efficiency)) × Ability Strength

For this build it would be:
1 + (25 × 15% ÷(2 − 90%)) x 400% = 76
76 energy for every killl
Headshot kills are a x4 multiplier so:
76 x 4 = 306 energy for every headshot kill

Heres a test build with 400% Ability Strength Test Harrow 400% Abiltiy Strength for reference

I have 3 Archon Shard Slots open. I'm thinking of going with 1 for shield, 1 for armor and 1 for casting speed.

1 Azure Tauforged shard is 225 units which eqates to about 2%?

I added a few mods to get close to +225 armor so its not even worth it.

I don't know if [Lasting Covenant] is worth it. If not then [Rolling Guard] or [Steel Fiber] could replace it. The pro is that I don't need to cast Covenant again so I can cast other abilities instead. The con is that I seem to make so much energy that recasting Covenant seems trivial.

I don't think the 20% lifesteal from Penance is a good enough buffer to rely on. Its only 214 health per hit so with this Akvasto build (https://overframe.gg/build/464233/akvasto-prime/akvasto-for-harrow-prime-gunslinger/) you would have 6.49 fire rate. Which sounds like a lot. I don't know the actual math for it but I still rather have another layer of defense including Pillage.

Penance gives a 232% Reload Speed Buff and that version of [Akvasto Prime] has 1.4s reload so I guess you instantly reload?

The Lifesteal Buff sounds like its a good way to always have health but not to rely on it.

Old Notes that I have to adjust for this guide

Theres a few variations so I'll go over them. Ideally you want 305% Ability Strength. Molt Augmented gives 60% and then you cover the rest with the Archon Shards so that would be

If you run [Power Drift] then your Ability Strength is 275% so you only need 2 Crimson Tauforged Shards or 3 regular shards for 305% Ability Strength.

If you are not running Powet Drift then you need need 3 Crimson Tauforged Shards or 4 regular shards for 305% Ability Strength.