Hello there,

As per Wiki:

• Tenet Flux Rifle: Enhanced fire rate and range, but uses clip magazine instead of an auto rechargeable battery.
• The weapon's max rank caps at 40 after 5 polarizations - max rank increases by 2 per Forma added (similarly as the Paracesis).
• All Tenet weapons deal innate elemental damage in addition to physical damage. The amount of this damage is random, ranging from 25-60%, while the type of element is determined by the Progenitor Warframe, as per the table below:

According to the above, I added exactly 5 forma to this weapon, so we can obtain Mastery Points from it.

In this case, I recommend picking Toxin as an elemental.

Some info about weapon itself:

• Pinpoint accuracy.
• Very high fire rate.
• Large magazine size.
• Large reserve ammo.
• Fairly fast reload speed.
• Below average critical multiplier.

This is a Hit-Scan rifle so accuracy isn't the problem here. Worth to mention this is also a beam rifle.
Thanks to high slash damage we can take advantage of Viral elemental, and because this weapon is fast and has nice Critical Chance/Multiplier we can push that damage even futher with [Hunter Munitions].
Overall, it's a pretty fun rifle to use.

Enjoy, THeMoon85