Hmm... Advanced Scripting, huh? Next time choose the General Editing board.
To make flak explosions just camcreate two shell (or whatever explosives you use) at the same time and same place - they'll detonate eachother.
To blow up a chopper: just setdammage 1 (basic) or camcreate two shells (again) at the chopper's place to give some realistic effects.
_shell = "Shell73" camcreate [x,y,z]
z can be, for example, (position chopper select 0)
Note: 'position' requires Res; otherwise use getPos