Literally the same thing as my riven build for this weapon: https://overframe.gg/build/252886/exergis/overpowered-riven-build/
If you can, try both, and see if you can tell a difference in actual performance.
(I sure can't - I'm pretty sure the riven I use is pure overkill for basically everything.)

The [Exergis] is a vastly underrated weapon. Even without the riven, its perks and unique properties make it much more powerful than advertised.

Projectile Properties:

It is projectile based, meaning it works well with Mag's bubbles and interacts in really interesting ways with the mini mag bubbles created by void damage / Xata's Whisper.

It technically has infinite body punch-through, and innate 0.5 meter hard surface punch-through.
This, again, makes it really useful for [Mag] bubbles of any kind, and means shooting it down a hallway full of enemies will see you hit every one of them standing in line of the shot.

Innate Radiation means enemies will attack each other, and it is especially good against robotics.
High base slash means it's easy to get bleeds to proc.

Weapon Properties:

Most weapons have a 0.5 second delay before the reload happens when you hit the bottom of the magazine. By default, the [Exergis] only has one shot, so they removed that for this weapon. Meaning the 0.7 reload you see above actually happens in 0.7 seconds, not 1.2 seconds like other weapons would.

High Status and Low Crit makes this an excellent status weapon. Don't waste time packing crit mods on here unless you have some way to boost it to meaningful levels. ([Harrow] mostly)

By default it should fire 3 rounds per second, if you somehow get more than a couple of rounds into its chamber. For this reason, fire rate mods are entirely useless, since it already fires much faster than you'll ever get the reload speed to.

My Build:

Adds heat and viral to the mix, and a strong emphasis on reload speed, multishot, and status chance. You don't need +base damage with this build since we're already getting a ton from Blaze, Galv Savvy, and the Merciless Arcane. The extra punch-through is there more for QoL than anything - it keeps the projectiles from vanishing too quickly into walls when used by [Mag]'s magnetize. Feel free to replace it with someting else if you wish.

I've been testing it in the simulacrum (ballroom) against level 155 Battalyst Sentients while running [Xata]'s Whisper, and it can almost demolish the whole crowd of 20 in two shots if they stay close enough together. The orbit effect of the mini mag bubbles from void damage makes the projectiles from this weapon have an almost chain-lightning effect through crowds, as nearly each projectile will proc void status and immediately start orbiting the target, but the orbit is quite wide - wide enough to clip other nearby enemies.

I've seen the same shard bounce between two enemies dealing damage to both over and over again because of this effect.

[Mag] capacity vs reload speed:

You should always try to have as much reload speed as possible, up until you hit around 0.8 or so. Really anything less than 1/second is good enough, more if you feel like it. It'll be hard to tell the difference and you'll start getting diminishing returns beyond that point anyway. For this reason, you can replace Chilling Reload with a normal 60/60 cold mod, or with the 60/60 electric mod if you want corrosive.

The riven shown on my riven build has extra [Mag] capacity. If yours has reload speed, or if you're using the rivenless build, then I highly recommend putting [Ammo Stock] in your build. Overwrite [Seeking Force] with [Ammo Stock] if you have a riven, or put it in place of the riven if you don't. The fire rate of this weapon is 3.33 which is MUCH faster than your reload speed will ever be. Having an extra round in the chamber is AWESOME for your DPS. You can technically push it to get up to 2 more rounds in the chamber, but it's not worth it due to diminishing returns.