I don't personally use sniper rifles for Eidolons but this is by far the lowest investment primary to get the job done. With all buffs up (Shock Trooper, Eclipse, Vigorous Swap, Electric Shield, sniper combo) one/two-tapping limbs and capshots is trivial.

The disposition is a paltry 0.60 so the main benefit from a riven would be >= 50% multishot to guarantee three pellets - both for damage consistency and building combo. This can be met on a +++- riven - other desirable stats are crit damage, damage, heat fire rate, and crit chance.

• Amalgam Serration is essential because the sprint speed affects Archwing.
• Critical Delay at rank 4 reaches > 100% crit chance with a tad less -fire rate than maxed.
• Vital Sense for critical damage.
• Split Chamber and Vigilante Armaments for multishot.
• Vile Acceleration for fire rate to build sniper combo faster and for faster limb breaks.
• Primed Shred for more fire rate and to help with Gantulyst orbs (replace with Speed Trigger if you don't have this mod).
• Hellfire for heat damage, combines with Shock Trooper to give radiation damage.
• Stabilizer for less recoil, exilus is mostly preference though.
• Primary Merciless at max rank gives +30% reload speed and +100% ammo max (+ammo max removed in U32), which is very helpful.