Good start on this script. To get right to the point, here's what I think should be added/changed.
1. First off, since you only create one shell repeatedly, it creates the effect of either one super-fast gun or a battery that fires a gun every .3 seconds. I use variables in scripts like this to create a volley of shots, then a reloading wait, then another volley. The effect looks really nice. Also, it's unlikely that you'll be subjected to a great deal of really heavy artillery fire. An effect that looks really nice is huge amounts of small caliber stuff (heat73s for example) to simulate mortars, and a small, slowly firing group of heavy guns.
2. Second, the wind isn't causing the shells to strike to the north-east of your target, it's the nature of the "random" command. Since you're creating the shells at locations from an X,Y array, and since use of the random command only produces numbers between 0 and whatever you specified (And you always specify positive numbers), the shells must strike to the NE of the target. This is because both the X and the Y coordinates have a positive number added to them. Since adding to X moves the position to the right/east and adding to Y moves the position up/north, you will always have shells land NE of your target. Add a lot more randomness to the coordinates to fix this, and subtract as well as add!