there is an artillery script out there that does just that already. Try looking in the tutorial section of this site, ill try and find the d/l i have of it, and post it.
You have to use the radio and place markers on the map where you want the barrage of fire. There's also a neat grenadier script that does a bombardement to a gamelogic etc maybe modigy them for your own needs.
I personally simulate artillery with a radio trigger gamelogics and time delays, its crude, but does the job. Damn I have to get mission editing again....