I couldn’t find a write-up on the damage thresholds online. So, here they are up to the Guard for all your damage vs ammo dilemmas and general convenience! All are hazard 5 scaling:

Swarmer: 15

Exploder: 24

Web Spitter: 47.88

Grunt: 108

Acid Spitter: 144

Slasher: 177.6

Septic Spreader: 324

Guard: 324

Happy hunting, and rock and stone!