Welcome to this guide, where I will take a look at the [Kuva Zarr] and share my thoughts, opinions and choices for modding this weapon. The [Kuva Zarr] is currently one of the best, if not the best AoE weapon in the game. You can get one by vanquishing a Kuva Lich that has spawned with one, and it also has an innate elemental bonus that depends on the lich's progenitor warframe (more information on liches can be found here). For this build I will be using a toxin elemental bonus.

The Kuva Zarr has two fire modes:

• Cannon mode fires a single cannonball, which explodes upon hitting anything, separating into three smaller bomblets, which also explode when they hit anything.

• Barrage mode turns the weapon into a shotgun which fires ten flak projectiles.

This build concentrates on the Cannon fire mode, as while Barrage can deal substantial damage to single targets, the meta right now favors explosive AoE weapons to quickly clear crowds, and complete missions in a fast and easy manner.

What this build wants to accomplish:

This build is meant to be an efficient and effective late-game weapon that we can comfortably bring into almost any mission in the game. This means that we will try to use the best available mods, but no fear: this guide will also give you alternatives in case you haven't progressed enough into the game to obtain everything.

On forma-ing Lich weapons:

Kuva/Tenet weapons (and some others) have a mechanic where each time you apply forma to them, their [Maximum Capacity] increases by one (two if you have an Orokin Catalyst applied), up to five times. This means that, while 3 forma are enough to fit this build, I highly recommend using the full five forma so you can obtain the maximum mastery from this weapon.

A quick look at the [Kuva Zarr]

To have an understanding of what we're working with, we can head over to the wiki page and check out the base stats on an unmodded Kuva Zarr.
The cannonball and its explosion share the same stats:

-25% critical chance: good, this allows us to use Hunter Munitions

-2.5x critical multiplier: very good. Critical multiplier is a much more valuable stat than critical chance, as there are not many ways to increase it outside of a few mods.

-31% status chance: also pretty good. This means we can reliably proc status effects.

-7 meter explosion radius: worse than the [Kuva Bramma]'s 8.3m and the [Tenet Envoy]'s 8m, but the [Zarr] has a better falloff than these two, going down to 70% from the Envoy's 80% and the Bramma's 90% (for the uninitiated, falloff is the amount that damage decreases as you get further from the explosion's center).

These are all-around great stats that push us towards a Viral + [Hunter Munitions] setup, but it gets better: each cannonball also spawns three additional bomblets! These deal much less damage than the main explosion, and have a much smaller 3m radius, but they have the same critical and status chance. The number of bomblets spawned by each shot is also influenced by multishot.

The recharge on the [Kuva Zarr] works similar to the original [Corinth]: only one cannonball is reloaded at a time, but you can interrupt it at any moment to fire. This means that while a full reload takes a whopping 4.8 seconds, a partial one can take much less. This makes the Zarr much more suited to compulsive reloaders than to people who run around with a half-empty magazine. I personally haven't felt any necessity to add reload mods.

Mod Choices With Explanations

Here I'll go over the mods I used in the build, and I'll try to explain why I think each mod is important, and what other mods can be used in its place if you don't have it.

Fundamental Mods

These mods are necessary, full stop. They are extremely strong and essential to the build, and have priority over all others. All of these are used in many other builds.

[Galvanized Chamber]: a fundamental damage mod. Multishot increases not only the number of shots fired, but also the number of bomblets spawned by each shot, which makes it an essential stat for our build. You can get this mod by trading vitus essence with the Arbiters of Hexis, but this requires access to Arbitration. If you can't access Arbitration yet, you can use a regular [Split Chamber] or buy one from another player for ~15 plat. I'd highly recommend getting this mod in any case, as it is used on almost every primary, and for good reasons.

[Critical Delay]: Another essential damage mod. We don't really care about fire rate on an explosive primary, and the 200% bonus critical chance from this mod is too good to give up. You can farm this from Orokin Vaults on Deimos, which are easy enough to do with any invisibility/immunity frame (an good one to get early on is [Limbo]). Farming this will also get you many other essential mods in the process, and you can also scan Kavats while you're at it. You have no excuse not to own this mod.

[Vital Sense]: The best critical damage mod we have access to for primaries at the moment. You can get this mod fairly easily from various regular starchart missions; check out its wiki page for more information.

[Hunter Munitions]: our last fundamental mod. This is extremely important to scale the build into later levels: slash procs add considerable DPS and completely bypass armor, making them our main way of dealing with tanky units such as the Grineer. With this build, since we have 3.9 multishot and 75% critical chance, [Hunter Munitions] will proc slash on 76% of all targets on the first shot, but it can also proc on the bomblet explosions, which are many. You can get this mod from Ghoul Bounties or other players, and frankly it has no substitute. Enjoy the farm.

Less Fundamental Mods

These are less important, and can be tweaked according to your preferences. I still think they're better than most others, however.

[Primed Cryo Rounds]: the only elemental mod in this build, this combines with the toxin from the progenitor bonus to form viral. Why viral in particular? Because it synergizes with the slash procs from [Hunter Munitions], increasing their damage substantially. For more info on viral status effects, read here. Why not form viral using two mods? Simply put, elemental mods are multiplicative with base damage and crits, but additive with each other. Therefore adding a second mod to form the combination element is less efficient than running only one elemental mod which combines with the innate bonus, as we incur in diminishing returns in the first case. So we run the mod which gives us the best possible bonus for the best possible elemental combination, which is [Primed Cryo Rounds]. If you don't have this mod, you can either get it from Baro Ki'teer if he brings it in his shop, which is available every two weeks, or you can buy it from other players. While you don't have it, a normal [Cryo Rounds] works fine.

[Primed Firestorm]: this is a mod that I rarely see on explosive builds, and honestly I don't understand why. It has great utility: according to the wiki, it increases blast area by 175.6%, which means with each shot you're hitting almost three times the enemies you'd hit normally! Furthermore, increasing the range also reduces the fallout of the AoE: enemies inside the original area will end up taking up to 28% more damage. Finally, this also increases the explosion radius of the bomblets, making it much more likely that their AoEs will overlap and deal damage (and apply status effects!) to multiple targets. The only downside this mod has is that it's useless for barrage mode, but frankly the upsides are worth it in my opinion. If you don't have this mod, I personally wouldn't run the non-prime version, as the 30% bonus is a bit low. I'd rather run a raw damage mod such as [Serration] or [Heavy Caliber].

[Hammer Shot]: this gives us a bit of critical damage, which is nice, but it's mainly here because it provides a decent amount of status chance. With only this mod, our status chance is at 55.8%, which means that with our 3.9 multishot we will be applying at least one status effect 96% of the time with just the initial shot! And if that's not enough, the swarm of bomblets that will explode soon after (with the same 55.6% status chance) will all but guarantee a multitude of procs. Viral is the main element in our explosions, so it will be the most frequently applied status effect. This mod is the most difficult to get for this build, as it drops as a mission reward from rotation A Nightmare missions, with a 1.01% chance. If you don't feel like farming it out or buying it from another player, you can comfortably run a regular [Rifle Aptitude].

[Vigilante Supplies]: the [Kuva Zarr] has a fairly low magazine size, so some form of ammo mutation is essential, especially in high level missions. Vigilante Supplies is our go-to, as beyond solving our ammo problems, it also has a chance of upgrading our crits. If you run a sentinel weapon with more mods from the Vigilante set , this can become a fairly consistent increase in DPS: personally I use this. You can get this mod from Cetus bounties, but if you don't feel like getting it, a normal [Rifle Ammo Mutation] works fine. I personally don't think the primed version is worth the cost in credits and endo to upgrade it.

[Vigilante Armaments]: the last and most debatable mod, this is here since multishot is incredibly strong on the [Kuva Zarr], and it also pumps up the Vigilante set bonus. If you feel you have better alternatives, feel free to replace this. An excellent mod for this slot would be a (Primed) Bane mod ([Bane Of Grineer], [Bane Of Corpus], etc...). Bane mods provide incredible damage increases, as they increase not only the initial damage, but also the damage applied by status effects, like the slash DoTs we're getting from Hunter Munitions. Since these DoTs also scale on the initial damage, this makes bane mods effectively double-dip in multiplicative damage bonuses. Why am I not running them? Very simple: I don't like changing my build before each and every mission, and I don't feel like spending hundreds of thousands of endo and tens of millions of credits on upgrading these mods. To be honest, I think they're a terrible design choice, however there's no denying that they're extremely effective. If you're intent on running extremely high level content, such as long steel path survivals or endurance runs, or if you want to extract the maximum possible damage from your weapon, these are the best choice for this slot, hands down. For all other regular and steel path content, you can do just fine without them. Alternatively you can go for [Bladed Rounds], or a Riven if you have a good roll. The best positive stats are Multishot, Critical Chance, Critical Damage, Status Chance, Cold, and Damage, in that order. Good luck though, as the [Zarr] has the minimum riven disposition, and I don't see it going up any time soon.

What mods are notably missing?

[Serration] and [Heavy Caliber]: these are two very strong mods that I decided not to run, and for a very simple reason: we are already running the Primary Merciless weapon arcane. This arcane, fully stacked, gives us a a 360% damage boost, more than both of these mods combined, and it stacks additively with them. This means that if we run Primary Merciless, these mods will give us greatly diminishing returns, therefore there are better options for those slots. More information on damage calculation can be found here. Since weapon arcanes drop from Acolytes, if you haven't finished the Starchart and you can't access Steel Path, then you should run one or both of these mods.

[Galvanized Aptitude]: a very strong mod that would be perfect for this build, if it worked with AoE damage. Unfortunately, the damage bonus only applies to direct hits, not explosions. I'm as sad as you are. However, if combined with a primer such as th [Kuva Nukor] or [Epitaph], this can be an excellent mod for a build focusing on Barrage mode to kill bosses such as Kuva Liches or Acolytes in a couple of shots.

[Galvanized Scope]: this mod relies on a particular playstyle, where you use Barrage mode to get headshots and then unleash the pain in Cannon mode with your increased critical chance. However this is high maintenance, and not my preference. I do not like to aim my explosive weapons.

Cheap and easy build for newer players

A simple and easy build that doesn't require any hard-to-get mods or arcanes. This will easily get you through most starchart, sortie or arbitration missions, but you will probably struggle in Steel Path. At that point I would start working towards the full build.

Damage Test

As you can see from the video, the damage takes a moment to ramp up as we stack up our Galvanized Chamber and Primary Merciless, but once we hit max stacks we're 1 to 2-shotting level 170 Corrupted Heavy Gunners. Barrage mode also works great, even with an unused mod slot ([Firestorm]), as you can see. All in all very satisfying weapon.

Conclusions

I love this weapon, and with this guide, I hope you will love it too. If you have any suggestions, feel free to leave a comment. If you enjoyed this guide, help make it visible to others by leaving an upvote!

Enjoy the explosions,
SilverHalf