Index focused budget build

All of the enemies we face in The Index have standard Corpus Shields and Alloy Armor. Corpus Shields are weak against Impact Damage (natively provided by the weapon) and the Alloy Armor is weak against Radiation Damage (which deals +75% damage to Alloy Armor).
Since the Alloy Armor decreases damage significantly, we need to focus this build against Alloy Armor.

Add Radiation Damage

So we can wreck those shields

Increase Damage

Add multishot (+180% total), Fire rate (+60%) to pump up DPS and [Hornet Strike] to increase global damage.
Finally Critical chance (+120%) and Critical damage (+60%)

Possible enhancement

You could add a forma to be able to put primed version of [Hellfire] or [Target Cracker]