Felarx magazine capability?

[BlackCatticus]

I have a maxed Felarx shotgun with an evolution III quest to get 20 headshots on a void Angel with out reloading. My build includes Maxed: Ammo stock, Burdened Magazine, and Atomic Fallout .  This means an increase in the ammo magazine up to 16 which still falls short.   I looked up on the internet and it only shows the three I currently have,  Is there other Mods to increase a shotgun ammo?

[Tiltskillet]

Pellets count individually for headshot hits, so you don't need huge magazine size. 

I did it with a basically unmodded  Felarx except for Hell's Chamber and Vigilante Armaments, the idea being I didn't want to kill the angel before the finishing the challenge.  And I used my sidearm after that to finish him off.