The [Fulmin] is a really versatile weapon, capable of specializing against Corpus (Magnetic Damage) and Grineer (Corrosive Damage) while only swapping out a single mod. It can also be quite useful in The Index, as I will illustrate here. This build is a is a relatively easy-to-build [Fulmin] setup designed to excel in The Index. In this guide, I will provide a brief description of the goals we need to aim for, and which mods we will use to attain them. I will also provide a brief description on how the weapon can be used and an estimate of the actual damage dealt (which will be higher than displayed by Overframe).

GOALS

We know that all of the enemies we will face in The Index will have standard Corpus Shields and Alloy Armor (which you start hitting after you bring down the shields). Corpus Shields are particularly weak against Impact Damage (which deals +50% damage to Corpus Shields and is provided by the semi-automatic fire mode. However, since these Shields are relatively squishy, we cannot focus this build on taking them down), while the Alloy Armor is exceptionally weak against Radiation Damage (which deals +75% damage to Alloy Armor) and somewhat weak against Puncture Damage (which deals +15% damage to Alloy Armor and is provided by the fully automatic mode). Since the Alloy Armor decreases damage significantly, we need to focus this build against Alloy Armor. The only required damage type not natively provided by the weapon is the Radiation Damage, but it can easily be added with a single mod. Asides from adding Radiation Damage, we wish to maximize the overall damage per shot, in addition to the burst damage (i.e. by increasing the fire rate).

Finally, we also wish to play to the weapon's strengths, which are primarily its very high critical chance of 30% (28% in full auto) and critical damage.

To sum everything up, we wish to:
1. Add Radiation Damage
2. Increase overall damage by adding critical chance, critical damage, and plain damage boosts.
3. Increase fire rate

MODS

Here I will illustrate how we will accomplish the aforementioned goals.

Add Radiation Damage

Since Radiation Damage is a combination of Electricity (already provided in both fire modes!) and Heat, all we need to do is add *[Hellfire]**.

Increase Overall Damage

To increase the frequency and damage multiplier of critical hits, all we need to do is add [Point Strike] and [Vital Sense], respectively. The frequency of critical hits is further amplified by [Split Chamber], which has a 90% chance of adding another shot (for which the critical chance is rolled again!) at no extra ammo cost.

To increase the actual damage, we can simply use [Serration]. At rank 10, it provides a 165% boost to damage.

Increase Fire Rate

This is done simply by adding [Speed Trigger].

Alternative Build

If you do not wish to waste a forma on the weapon, or if maximizing these mods (especially [Serration] is too expensive, you can keep [Serration] at rank 8, and keep [Hellfire] at rank 0. However, this will severely limit the functionality of the weapon (the burst damage is essentially halved!).

Once you do forma the slot for [Split Chamber], the first mod I would upgrade is [Hellfire], since it will provide the highest damage boost, and will cost considerably less compared to maxing Serration. The endo cost by maxing Hellfire and not Serration is only 8,510 endo.

Damage Calculation

Semi-Auto on Shields

The Impact Damage will receive a 50% boost against Shields, so the base Impact Damage is increased to 265.0 * 1.5 = 397.5. The Radiation Damage will receive a -25% modifier against Shields, so the base Radiation Damage is decreased to 0.75 * 2,252.5 = 1,689.375. This results in a base damage per shot of 2,086.875.

Now, we need to take into account the multishot, critical chance, and critical multiplier. From the multishot and critical chance, we can expect to get 1.9 * 0.75 = 1.425 critical hits per trigger pull. So, from critical hits, we will expect to get 1.425 * 4.8 * 2,086.875 = 14,274.225 damage. From non-critical hits, we can expect to get (1.9 - 1.425) * 2,086.875 = 991.266. So, the average amount of damage to Shields we can expect to obtain from a single shot is 15,265.491 damage.

Therefore, by multiplying the fire rate by this damage value, we obtain a burst DPS of 3.47 * 15,265.491 = 52,971.252 DPS. Alternatively, we obtain a total damage of 6 * 15,265.491 = 91,592.944 damage without recharging.

Semi-Auto on Alloy Armor

The Impact Damage will not receive any buff against Alloy Armor, so the base Impact Damage remains at 265.0. However, the Radiation Damage receives a +75% boost against Alloy Armor, so the base Radiation Damage is increased to 1.75 * 2,252.5 = 3,941.875. Therefore, the base damage per shot is 4,206.875.

Again, we need to take the multishot, critical chance, and critical multiplier into account. As before, we can expect 1.425 critical hits per trigger pull, so we will expect to get 1.425 * 4.8 * 4,206.875 = 28,775.025 damage from critical hits. From non-critical hits, we can expect to get (1.9 - 1.425) * 4,206.875 = 1,998.266 damage per shot. So, we can expect to obtain 30,773.291 damage per trigger pull.

So, as before, we can obtain the burst DPS: 3.47 * 30,773.291 = 106,783.318 DPS. Alternatively, we can calculate the net damage without recharging: 6 * 30,773.291 = 184,639.744 damage without recharging.

Full Auto on Shield

The Puncture Damage receives a -20% malus against Corpus Shields, so the base Puncture Damage is 0.8 * 21.2 = 16.96. The Radiation Damage receives a -25% malus against Corpus Shields, so the base Radiation Damage is 0.75 * 145.0 = 108.75. So, the base damage per shot is 125.71.

The critical chance and multiplier are different for the full auto mode than for the semi-auto mode. So, we can expect 1.9 * 0.7 = 1.33 critical hits per shot. Therefore, we expect to get 1.33 * 5.3 * 125.71 = 886.130 damage from critical hits in a single shot. From non-critical hits, we can expect to get (1.9 - 1.33) * 125.71 = 71.655 damage per shot. This results in a net damage of 957.784 damage per shot.

We can calculate the burst DPS: 14.93 * 957.784 = 14,299.722 DPS. The damage without recharging can also be calculated: 60 * 957.784 = 57,467.069 damage without recharging.

##Full Auto on Alloy Armor
Puncture Damage receives a 15% bonus against Alloy Armor, so the base Puncture Damage per shot is 24.38. Radiation Damage receives a 75% bonus against Alloy Armor, so the base Radiation Damage per shot is 253.75. So, the base damage is 278.13 damage per shot.

As before, we can expect 1.33 critical hits per shot. So, we can expect to get 1.33 * 5.3 * 278.13 = 1,960.538 damage from critical hits, and (1.9 - 1.33) * 278.13 = 158.534 damage from non-critical hits. The net damage per shot is 2,119.072.

We get 14.93 * 2,119.072 = 31,637.752 DPS and 60 * 2,119.072 = 127,144.348 damage without recharging.

How To Use The Weapon

From the above damage analysis, it is better to use the semi-auto mode in most situations. In addition to dealing a higher burst DPS, the semi-auto mode also has an innate punch through (and is able to hit multiple enemies in a strait line) and has the additional benefit of guaranteeing an Impact proc to any enemy hit within 10 meters, staggering them for 1 second.

However, it also chews through ammo considerably quicker consuming a single magazine in a meager 1.729 seconds, compared to 4.019 seconds for the full auto mode. Additionally, the projectile produced by the semi-auto mode travels considerably slower and has limited range. So, when engaging a single target at long range, it is probably better to use the full auto mode.