Set your Trigger Zone's AIMAIN to plrinzoneactivateused.fpi (it's a stock script), and then put the decals name in the trigger zone's IFUSED field.
Then try this
;Artificial Intelligence Script
; 0-once face player
; 1-once keep angle
; 2-loop face player
; 3-loop keep angle
; 4-once face up
; 5-loop face up
; 6-character-spot-decal (at end of entity characters gun if available)
;Header
desc = Runs Decal Once Then Destroys It
;Triggers
:state=0:activate=0,state=1
:state=1,activated=1:rundecal=0,state=3
:state=3:runfpidefault=2
;End of Script
for your Decal's AIMAIN.
I modified it from a stock decal script, and it has been tested with the stock fire.
Let me know if it works for those pack decals.