Each Archgun does a different job. Most don't do it well, but this one does. This particular one is best for one shotting The Jordas Golem, Eidolon Limbs, the Profit Taker's Legs, and other tough enemies, provided you build it correctly. This particular setup is best for The Jordas Golem, as he is weak to Corrosive Damage. The Jordas Golem's armor (Ferrite Armor) gets a +75% damage increase from Corrosive Damage. Since he has 73.17 Damage Reduction from armor, think of this as a ~2% damage increase, whereas all other damage would be reduced. His health (Fossilized Health) also gets a +75% damage increase from Corrosive Damage. For Eidolons and Profit Taker, use this same setup but change the elementals to Radiation (Heat and Electric, but use [Magma Chamber] in place of [Contamination Casing] because [Volt] Shield gives Electricity Damage).

TLDR: This build one shots stuff. For PT and Eidolons use Radiation (with [Magma Chamber]). Always use charged shots (though they will stagger you if you deploy it as a Warframe).

Now, I'll take you through my reasoning on the mods.

1) Unless you're using Chroma for Base Damage (don't recommend. Chroma kinda sucks now) Primed Rubedo-Lined Barrel is the single strongest Archgun mod in the game. If you don't have it, your next best option is Rubedo-Lined Barrel. If you are using Chroma, remove (Primed) [Rubedo-Lined Barrel] and replace it with [Charged Bullets] for even more Corrosive Damage (or build for Radiation for Eidolons or Profit Taker)

2) [Dual Rounds]: This is multishot. This always goes on every gun. Why? Multishot has the least amount of diminishing returns in the game. A gun that fires 2 shots essentially gets 2 times total damage (after mods) and 2 times status procs. Dual Rounds is mandatory just like (Primed) Rubedo-Lined Barrel.

3-5) Critical Mods: Critical mods are mandatory on any gun you use for Status Immune enemies (Profit Taker and Eidolon) or for big damage. When combined with [Odonata] (Prime)'s first ability (the shield), you gain an additional 2x critical damage. So Critical Chance is mandatory. As for Hollowed Bullets, this becomes mandatory when using Critical Chance mods.

6-8) Elemental Mods: These are interchangeable, depending on the enemies you face. If you are killing the Jordas Golem (which this gun is best suited for) use this mod configuration for the reasons I explained above. If you are facing the Profit Taker or an Eidolon, use [Combustion Rounds], [Electrified Barrel], and [Magma Chamber]. In Railjack missions, secondary damage types (combined elements) do not proc but do count for a damage bonus. Since most Grineer fighters use Alloy Armor, use Radiation Damage, and change [Contamination Casing] to [Polar Magazine] (in the event you don't one shot a fight you will Cold Proc it, they will be frozen in place for the follow up shot). For Corpus, use Radiation since most Corpus fighters also use Alloy Armor. The base Magnetic Damage from [Velocitus] will make quick work of their shields (blue health) as well. Cold is still the best primary elemental (single element) to use because of its proc.

Sub-Optimal Mods

1) [Archgun Ace]: requires headshot kills. Since none of the enemies you fight (except Eidolons) have a head, this mod is essentially a wasted slot. Unless you use this weapon for normal enemies, it will do no good. Archwing enemies and Railjack fighters have no head either. Even on Eidolons, the headshot is the kill shot. Vomvalysts do count as a head, but since they must be killed with an amp, this still doesn't help.

2) [Deadly Efficiency]: Requires you to reload from empty. You should never have to reload before the enemy is dead, so this mod is also worthless.

3) [Shell Rush]: This mod isn't bad. If you are fighting regular enemies or Archwing enemies, it is actually better than using a Dual Stat mod (like [Magma Chamber]) both for QoL and DPS. However, most of the use cases for this gun are against slow moving enemies where you need all the damage you can get. I do recommend using this mod while leveling the gun.

4) [Sabot Rounds]: Punch Through is always a good stat to have. If you are grouping enemies up or lining up your shots, the DPS increase from this mod is amazing. However, we use this weapon to do big damage against boss type enemies, so punch through isn't really helping. Additionally, since this weapon is a projectile, it cannot do water limbs on Eidolons, even with Punch Through.

5) [Quick Reload]: Reduces the delay for ammo regeneration in Archwing. On the ground, it does exactly what it says. Again though, you shouldn't be reloading often before the boss dies, so there is no real benefit to using this mod.

6-8) IPS (Impact, Puncture, Slash) mods: IPS mods are almost always bad because they only increase the base damage of their element and increase the chance to proc their element. These mods work more like (Primed) [Rubedo-Lined Barrel] than an elemental mod except for a single damage type. In rare cases, the increased chance to proc a certain status can be helpful. Anywhere you would realistically use this Archgun is not one of these rare cases. Furthermore, this Archgun is likely going to kill whatever it is pointed at, so status does not matter.

9) [Magazine Extension]: you don't need 6 extra bullets in the magazine for this gun unless you equip this mod. Reload speed will do more for you than this mod in every case except blasting low level enemies.

10) [Ammo Chain]: not terrible for using this on the ground, although you should have more than enough ammo to kill Profit Taker or Eidolons. If you are using this as a normal weapon in missions and don't have a way to get more ammo, use this mod instead of a Dual Stat mod.

11) [Automatic Trigger]: This mod does not do much for the fire rate of this gun. Charge Rate is the more important stat. Adding fire rate to a low fire rate gun is rarely helpful, especially when that gun has a low fire rate because it is a charge weapon.

12) [Ballista Measure]: you've never heard of this mod most likely. I personally think its a joke. It is never useful. There are 0 use cases for this. Do not use this mod.

13) [Marked Target]: adds a ton of status to a weapon. Using this and a Dual Stat mod, [Velocitus] can become a status monster. Too bad the enemy will be dead before a status happens.

14) [Modified Munitions]: This is a worse [Marked Target]. It also gives the same additional Status Chance as a Dual Stat mod for more drain.

15) [Resolute Focus]: honestly, this mod is good for QoL. When the [Velocitus] uses a charged shot on the ground, your Warframe will be staggered. The knockdown resistance and reduced spread are a nice bonus. If Archguns ever get an Exilus Slot, put this mod in immediately. Unfortunately for this mod, we one shot enemies, so a little stagger doesn't matter. You can also use [Primed Sure Footed] (or the Poise passive from Unairu) to ignore Stagger and Knockdown, so this mod loses even more value. Necramechs are immune to status, so it does not matter for them.