An [Afentis] build that focuses exclusively on the Ballistarii Might buff you get for throwing it on the ground. Since we won't be using [Afentis] for anything else, we mostly care about fire rate and reload speed (we even have a relevant but unnecessary Arcane if you have a spare adapter)... but we can't actually fill every slot in the build with only those stats, so I threw in some other mods that have some situational or stat-stick-related use. The arguments for those will be included in the explanations below.

Both of my [Afentis] builds actually focus exclusively on Ballistarii Might, because [Afentis] just doesn't seem to be worthwhile as a weapon. I've always wanted to like Spearguns, but even with that bias [Afentis] feels too clunky to use when there are just better Spearguns out there. The buff is definitely worthwhile, though, so the weapon isn't completely useless... just use-impaired. If that changes down the road, I might add a third config for use as an actual weapon... but I'm not holding my breath.

Mod Breakdown

[Terminal Velocity]: increases the velocity of not only its fired projectiles, but also the thrown spear.
Primed Shred: gives some additional fire rate, don't care about punch-through.
[Aero Periphery]: can put a few enemies to sleep if we fire the explosive bolt at them while airborne.
[Hush]: silences the weapon so it won't alert enemies.
[Amalgam Serration]: increases the base damage, but is really only there for the sprint bonus.
[Primed Fast Hands]: increases reload speed significantly.
[Vile Acceleration]: increases fire rate significantly in exchange for a damage decrease that doesn't matter.
[Speed Trigger]: another chunk of fire rate for the spear.
[Vigilante Fervor]: even more fire rate for the spear, and a set bonus that usually won't matter since the spear is a primary.

Mod Explanations

After slotting all the fire rate and reload speed that we can, we are left with four empty slots (including the exilus slot, only three without). Two of these slots are pretty easy to fill, but the other two... it's pretty much picking what the least bad or irrelevant option is? We don't care about damage mods or additional mods for the Vigilante set.

[Amalgam Serration] is a pretty easy pick, that bonus sprint speed is pretty much always welcome. We don't care about the damage of the weapon but we have the slot anyway.

[Terminal Velocity] is situational but also a fairly easy pick depending on your intended use case. I'm usually just throwing the spear at my feet and the extra velocity is hardly noticeable there, but it's reasonable to say it is useful when throwing the spear across a tile to buff a squadmate. This is also why we have [Primed Fast Hands] over something like [Tactical Reload], since we may not be throwing single isolated spears.

[Aero Periphery] isn't going to do a ton if it's your only mod out of the set since the sleep is so short, but the spear is explosive and can hit multiple enemies with one shot. If you choose to actively proc this set, it might be the only time you actually fire the spear.

[Hush] makes the weapon silent, and that probably isn't usually relevant... but there's not really anything better for the last slot and last few points of capacity.

[Proton Jet] is more expensive than the Aero mod, but if you like the set you could let it occupy the throne of irrelevance.

Alternatively, if you're planning to utilize the Aero set's sleep effect but are worried about self-stagger... you'd have to sacrifice Terminal Velocity (or add more Forma) but you could replace Hush with Cautious Shot to remove the self-stagger.

Alternate Builds

CONFIG B: Artemis Javelin, which lets you keep Ballistarii Might on hand but is otherwise a stat-stick for [Ivara]'s [Artemis Bow].