Exilus mod slot is open because none of them really matter, [Targeting Subsystem] would be the only one of value but not worth the use of a Exilus Adapter + Extra Forma.

Creeping Bullseye can be swapped with Primed Target Cracker, the fire rate decrease does not affect the weapon because of [Semi-Pistol Cannonade].

On weak point hit a maxed [Leaded Gas] mod will increase [Vesper 77] status chance to 104% for 6 seconds, thus guaranteeing gas cloud procs on subsequent shots.

Weak points for most enemies are their heads. But some enemies without clear heads, like moas, have weak points in other locations.

The color of the targeting reticle that appears on enemy head/weak points is dictated by the energy color of the weapon. Use a bright color to make hitting weak points easier.

The gas cloud is able to hit heads and other body parts by itself. However, the Gas Cloud only has a 1x damage bonus when headshotting. This can be increased with the use of Secondary Deadhead.

According to my tests having a higher base damage build to proc stronger gas clouds performed better agaisnt strong enemies rather than having a multishot build to get more gas procs even though the calculations state that multishot builds are supposed to have a higher DPS. This is why [Galvanized Diffusion] isn't present in this build.