In theory, these scripts will work for you.
In practice, they might be too complicated (I disagree, but some don't care how artillery really works).
oops. It turns out OFPEC has downtuned their file submission size. Hopefully soon I'll be able to post the artillery suite here. I can say for sure that in a couple days, it will be available at
www.thechainofcommand.net .
In any case, the easy answer is: make a trigger (set activation/deactivation as you see fit), that each time it fires shells the area
shell = "Shell120"; radius = 60; "shell camcreate [((getpos this) select 0) + (((_x mod 1)*radius)*sin _x), ((getpos this) select 1) + (((_x mod 1)*radius)*cos _x), 300]" foreach [random 360, random 360, random 360, random 360]
Shell120 may be a little beefy, so you can go smaller.