This is a fairly simple build; there's a line on Kinetic Bolt of Fragmentation that says ‘multiple of these projectiles can hit the same target'. I assumed this was referring to only the secondary projectiles, but no, it's certainly valid for the primary ones.

In short, Deadeye's 7 base projectiles get multiplied by 8, each, to result in 56 projectiles. They all can hit a single target, then split, then return, laying the smack down on single target, even with return support nerfing their damage by ~60%. This thing slaps single targets, and my base projectile DPS is only ~100k (PoB: https://pobb.in/dNTFsdREKjqo, still very basic gear, nothing special), which means my estimated DPS on returns is 56 x 100,000 x 0.38 = 2,128,000 dps.

The clear sucks; usually KB is good for that but this version really isn't. It helps to slap down a sniper's mark in a pack and have that take care of the clear (since the deadeye ascendancy moves the mark along between mobs for us, this is very convenient), but it sucks to have to mark every pack before shooting it. Mark on Hit won't work since it only applies to rares and uniques.

This could also work perfectly fine in ballistas, and perhaps even be better since you don't have to go into melee range to make the returns all hit.

As far as I know, this is one of the few (only?) projectile skills where the projectiles themselves can shotgun, as opposed to projectiles causing AoEs that can shotgun (Creeping Frost, new Caustic Arrow of Poisoning). That means there's probably more fun use cases for this skill out there, assuming this isn't a bug.