At least they've chosen a good weapon to deal with Parasytes - AA-12 full automatic shotgun. sadly they've chosen wrong rounds..
They knew that fire, and generally speaking burning works great vs Parasytes - why they haven't used HE ammo - which would've obliterated the guy who shot their bullets back at them.

Heck, showmanship Dragonbreath rounds would work well too - but guns might get damaged by it.