So you take the models and put them into the "limb finder" which should be in your Tools folder. Not sure what version of FPSC you are on but it comes with Black Ice Mod. Once you find the limb number you can hide the corresponding limb when the enemy takes damage to that limb with
Here is part of a script as an example I use this for zombies from zombie apocalypse 2 pack.
;HEADSHOT 1
:state=101:sethealth=900,incframe=13,hidelimb=52
:state=101,frameatend=13:state=102
;:state=102:sethealth=0,suspend,coloff
;HEADSHOT2
:state=103:sethealth=900,incframe=14,hidelimb=52
:state=103,frameatend=14:state=102
;:state=104:sethealth=0,suspend,coloff
;HEADSHOT3
:state=105:sethealth=900,incframe=15,hidelimb=52
:state=105,frameatend=15:state=102
;:state=106:sethealth=0,suspend,coloff
It looks good in action but the limb itself wont actually fly off the model it will just become hidden. However it does look pretty good and feel good when you shotgun a zombies head off.