This is just a basic build using common Mods that I already had from my previous gun (Lovely Basic Karak ^^) and is fairly standard all things considered- but requires no Forma and thus early-game friendly.

Note, this is all on a purely theoretical level as of right now and depending on how this plays out once my Soma Prime finishes in about 4 hours, this may be changed to more positive/negative

Main Pros of this build:

• Faster Reload speed than most other [Soma] Prime Builds (2.3 vs 3.0). I hate long reload times and this mitigates it slightly.
• Utilizes Mods that aren't overly Rare/Are fairly common and by the time of Mastery Rank 7, most players would already have
• Early-Game Friendly due to no usage of Formas

Main Cons:

• Isn't End-Game built and could probably Star-Chart clear with some effort but likely can't do End-Game Content without more meta combinations/Forma
• Requires an Orokin Catalyst to get to 60 Capacity (Although this is true of any and all weapons crafted)

Early Additions which can be done immediately is the exchanging of [Stormbringer] for [Cryo Rounds] as Slash Damage is already good against Infested and [Cryo Rounds] are good against both Armour and Shields, meaning both covers each other's weak points. [Stormbringer] being here is merely a preference.

Later additions can be the addition of 2 Madurai Forma to any of [Speed Trigger], [Point Strike], and Vital Sense (They're all 9 so it doesn't matter which two). This would enable you (and me, this guide is mostly for future-proofing for me lol) to play around with the elements more. All the text after this will assume you would have done the 2 Forma.
The Combination of Stormbringer + Malignant Force is viable and if time allows, High Voltage + Malignant Force as both produce the Meta-Corrosive damage.

For Higher-Level Content- the removal of [Speed Trigger] and replacement of its location with [Point Strike] (So you have Point Strike and Vital Sense in the Forma Slots) can free up 17 Capacity for more damaging mods. Assuming no access to any 60/60 elements, the combination of Stormbringer + Infected Clip yields Corrosive Damage- the Meta as noted above.

This also leaves you with 6 Capacity left, which can facilitate any of these mods depending on preference

• [Vigilante Armaments] (Rank 2; +30% Multishot and 5% chance to enhance CRIT DMG)
• [Bladed Rounds] (Rank 2; +60% CRIT DMG on kill when Aiming for 4.5s)
• [Hunter Munitions] (Rank 2; +15% chance to apply Slash Proc when CRIT is dealt)

and lastly, the Ideal Exilus would likely be [Vigilante Supplies]. While this gives a small chance to enhance CRIT Hits (5%), the main focus is its ability to turn any other ammo type into that for the Soma Prime- something that would be important for any drawn-out missions.

Why the lack of Heavy Caliber or Argon Scope?

Firstly- [Argon Scope]'s +135% Critical Chance on Headshot when Aiming is sort of overkill since our CRIT Chance is already 75% at a fire rate of 24.0

Secondly- Heavily Caliber's damage bonus is great but the -55% Accuracy is a hefty detriment as the weapon's accuracy is already fairly mediocre. I guess if you want a Close-Range-Shotgun-but-the-Shotgun-is-Full-Auto then go ahead (Even though there are better options for that, which all has the notable quality of being "Not Soma Prime"). The Endo Investment on Heavy Caliber is also hefty.

Thirdly- This build is meant for an early-game type player who's building as they go along- both Heavy Caliber and Argon Scope aren't early-game friendly.

Unless I specifically linked you this page, not sure why you're reading this but thanks for reading my ramblings for Future-me.