Several issues, not sure which are intended and which are not:
1) Does not damage or debuff Catapults. It affects all other magic immune targets.
2) Can be interrupted, setting off the cooldown and only throwing out 1 to 2 slugs instead of the full 3. It is not listed as a channeled skill and allows movement, so there really isn't a reason for it to be interruptible.
3) The area of effect changes based on whether Gunblade is facing the target or not when Crippling Slugs activates. The bullets can change trajectory by rotating during the cast time.
4) Cannot hit targets under the effect of Succubus' Mesmerize.
1)Catapults are also "mechanical" and was not targetable on purpose
2)Intended, it is built into the script
3)Intended (where else would the bullets come from! Its just that they curve!)
4) Replay and timestamp please, I'm not seeing this in-game.