This is the simpliest way that I have found.
_arty is the name of your artillery piece
_target is the name of your target (invisibleH's set in 500m hieght are best)
init field of your Arty piece
trigger activated by whatever you want that starts artillery firing:
example has 4 x BM-21 GRADS firing in sequence with 1 second in between each shot, after 5 seconds the battery repeats firing
copy and paste this into new script in your mission folder and name "gradfire.sqf"