Disclaimer

I do not guarantee the effectiveness of this primary weapon in high level mission such as Steel Path
But I can say that this is an OK Assault Rifle with quite the flexibility

How good is this?

The Kuva Karak as according to the Warframe Fandom wiki :

Advantage :

• Low recoil
• Innate punch through helps bypasses thin obstacles.
• Polarization capacity bonus allows for more flexible modding and easier use of high cost mods.
• Progenitor damage bonus provides extra elemental modding capabilities with the extra potency.

Disadvantage :

• None

Well I could definitely say the disadvantage of this is actually Zero AoE capability
But the 0.2 Punch Through means that Grineer Shielder has that thin piece of metal considered non-existent

Why a build for a non-META weapon?

I just like the Kuva Karak since it looks like a modern AR with a close shape to the M4/M16 family
Guns just looks cool ( You don't have to agree, it's just a personal bias )

Explanation

So the [Kuva Karak] has a really accurate bullet spread and a super low recoil, meaning if you aim it right on your reticule, you basically can shoot an enemy over 300m away with ease.

The low damage output is compensated with [Critical Delay] giving it a high crit chance and while [Vital Sense] and [Hammer Shot] gives a big Critical Damage Multiplier.

[Galvanized Chamber] gives multishot for more bullet output for every ammo used as we all know

[Galvanized Aptitude] gives a bigger status chance as well as more damage for every unique status applied to the enemy, which isn't hard to apply since...
You can see the status chance up there...
and after 2 kills, it will give an extra 80% damage inflicted for every unique status

Hunter Munition gives a chance to apply slash proc to the enemy when you crit which is basically most of the time, so it's not too hard.

Why the low fire rate?

I just suck at aiming so the slow fire rate conserves me a lot of ammo and help with the already low recoil to just stable.
I really am just horrible...
Also I don't know what to replace since this already do quite the damage.

Why in the world did you even need to put [Hush] on exilus?

I imagine as if the gun had a silencer on just to make it feel like it's cooler on that aspect.
Also helps during mission that require stealth like Spy if you want to make sure no enemy notice a thing.
It's still stealth if nobody lives to tell about it.
So go ham or whatever you desire on this case.
*Exilus is optional

Another alternative for the exilus slot is the [Vigilante Supplies] to give an extra 5% chance to enhance the crit damage you deal

Arcane choices

I primarily use Primary Dexterity since it also helps out your melee, and at the same time your melee kill also provides you a +60% damage bonus per melee kill up to 6x stack. So probably 480% damage bonus or however the game counts the extra damage multiplier.

In case of Sortie mission only allowing the use of Assault Rifle, probably change it to the other arcanes Primary Merciless & Primary Deadhead

Why Heat+Viral element?

Heat is just one of the best damage element out there in my opinion
Instantly reducing armor by 50%, a damage over time, and the enemy panicking, screaming as they burn for a while.

Viral procs gives the enemy a debuff of extra damage received of 100% for the first proc, and an extra 25% for each stack, giving it an extra 325% damage received at 10 stacks.

With extra damage received from Viral and a -50% armor from Heat you could deal big numbers of damage

Also I choose viral since that cold from [Primed Cryo Rounds] is preferred for it's high damage it gives, and that cold element needs to be mixed with something after all, and Viral is the best pick that comes to mind.

Final words

I do not think this is the best optimal build for the [Kuva Karak], but this build does justice for me
I hope you enjoy this build.

Regards, Chronosite