Is Zenith not a rifle?

[(NSW)AtomiKoala]

The Exlius mod terminal velocity states that it is a riffle mod.

The Zenith rifle can be equipped with other rifle exclusive exlius mods such as eagle eye.

The Zenith cannot be equipped with terminal velocity.

Am i missing something?

[(NSW)Belaptir]

Zenith is a hitscan weapon. It means its projectiles don't travel from the weapon to the place where they hit. They hit instantly. Since Terminal Velocity increases projectile speed and the Zenith projectiles don't have speed (you could say they teleport instantly) the mod won't work on it.

[(NSW)AtomiKoala]

A 'magic bullet' rifle? Kinda makes more sense when explained your way.

Is there any way to tell this from inside the game? 

As is, it kinda comes across as a bug in the game, not a feature of the weapon.

[Dark_Lugia]

This mod only works on weapons which have travel time projectiles (mostly bows, but also stuff like Buzlok, Supra, Tetra, etc.)
For hitscan weapons like the Zenith, Baza, Soma, Karak etc. you don't even have the option to equip the mod, since it wouldn't do anything.

(Projectile speed also influences the range of the dmg falloff, but this is mostly universal at 300m for hitscan weapons and often can't be changed, but on many shotguns it can be changed making fatal acceleration a nice exilus mod.)

[-AoN-CanoLathra-]

While it is true that the Zenith's primary fire is hit-scan, the radar disc launched by its alt fire is not, which begs the question of why terminal velocity can't be used to increase the flight speed of the radar disc.