Well.. in fact.. that mission did use an artillery script.... and you can find
tons of arty scripts right here :
http://www.ofpec.com/editors/snippet_cat.php?id=2most of the ones listed use a random function to make cam created shells fall randomly inside of a trigger's radius...
therefore increasing the trigger radius, and/or decreasing the number of shells will make the shells spread out farther on the battlefield, perhaps making it appear more random to the observer.
If one was so inclined... you could create a set of gamelogic points representing the impact points of your shells (i.e. g1-g8) and use this syntax in a trigger, or series of triggers with identical condition fields but offset by a few seconds utilizing the countdown funtion of said trigger.
b1="HEAT125" camcreate getpos g1
b2="HEAT125" camcreate getpos g2
this syntax could of course then be repeated ad infinitum
Plus the point is... who cares if you as a misson designer knows if they're falling randomly or not
creating the right timing and sequence of gamelogic targets, can yield a far superior effect anyways.