Code:

void main()

{

int FIRECHANCE = 50; // Percentaget chance of catapult to fire each 6 seconds

if (d100() < FIRECHANCE)

    {

    float dly = IntToFloat( d6() );  // creates a random delay so many catapults

                                     // does not fire at the same time

    dly = dly/2;

    DelayCommand(dly, ActionInteractObject(GetObjectByTag(GetTag(OBJECT_SELF))));

    }

}

the operator has a default 50% chance of firing the catapult each 6 seconds. can easily be changed if you want it to fire more often or seldom

This goes in the catapults OnUsed:

Code:

void main()

{

    // if (GetLocalInt(OBJECT_SELF, "CatapultReady") == 0)

    // {

        // edit these 4 numbers if needed

        float TARGET_RADIUS = 6.0; // Constant used to determine the radius of the fireball's blast.  Should be higher for Defenders than Attackers.

        int AOEDAMAGE = d20(2); // Area of effect Damage dice for the firedamage on impact

        float WAIT = 2.7; // delay between firing the catapult and creating the explosion.

        float MISSRADIUS = 10.0; // max offset in meters from target area creating miss effect

        // below does not need to be changed

        object oTarget = GetObjectByTag((GetTag(OBJECT_SELF) + "_t")); // Selects a target named after the catapult tag followed by _t

        // below looks messy but it just sets up random generator to create a range from neg to pos

        float MISSNEG = MISSRADIUS +1.0;              // dont touch =P

        int MISSRAN = (FloatToInt(MISSRADIUS) * 2)+1; // no touche! =P

        // vector calculation begin

        vector vselfpos = GetPosition(oTarget);  // get vector of current location

        vselfpos.x = vselfpos.x + (MISSNEG - IntToFloat(Random( MISSRAN )));

        vselfpos.y = vselfpos.y + (MISSNEG - IntToFloat(Random( MISSRAN )));

        location lTarget = Location( GetArea(OBJECT_SELF), vselfpos, 20.0);

        // vector calculation end .. lTarget is the new impact point with miss effect applied

        object oHarmed = GetFirstObjectInShape(SHAPE_SPHERE, TARGET_RADIUS, lTarget, FALSE, OBJECT_TYPE_CREATURE | OBJECT_TYPE_PLACEABLE); // Begins the process of damaging all creatures and placeables within a sphere of the target.

        //Function set that creates the fake fireball.

        ActionCastFakeSpellAtLocation(SPELL_FIREBALL, lTarget, PROJECTILE_PATH_TYPE_BALLISTIC);

        ActionWait(WAIT);

        ActionDoCommand(ApplyEffectAtLocation(DURATION_TYPE_INSTANT, EffectVisualEffect(VFX_FNF_FIREBALL), lTarget));

        SetLocalInt(OBJECT_SELF, "CatapultReady", 2);

        while (oHarmed != OBJECT_INVALID)

        {

            ActionDoCommand(ApplyEffectToObject(DURATION_TYPE_INSTANT, EffectDamage(AOEDAMAGE, DAMAGE_TYPE_FIRE), oHarmed));

            ActionDoCommand(ApplyEffectToObject(DURATION_TYPE_INSTANT, EffectVisualEffect(VFX_IMP_FLAME_M), oHarmed));

            oHarmed = GetNextObjectInShape(SHAPE_SPHERE, TARGET_RADIUS, lTarget, FALSE, OBJECT_TYPE_CREATURE | OBJECT_TYPE_PLACEABLE);

        }

   // }

   // else

   //     SendMessageToPC(GetPCSpeaker(), "The catapult is not yet ready to use again.");

}

the first number you might want to tweak is:
float WAIT = 2.7;
in the catapult script.. it determines the delay between fire and impact.
you might want to set it as high as 10.0 seconds if the catapult fires at extreme ranges. to time the actual impact of the fake fireball and the blast it creates.

Hope you have fun with it =P