Basically you get the spawn positions of the ammo like this:
_dir=getdir TANK
_dist=2
_spawnx= (getpos TANK select 0) + _dist*sin (_dir+_ang)
_spawny= (getpos TANK select 1) + _dist*cos (_dir+_ang)
_ang is the offset angle you add to the vehicle's map heading, i.e. how many degrees to rotate. Can't remember which direction positive values rotate, but I guess its to the left. The OFP coordinate system is totally weird though; it measures angles relative to the Y-axis.
Anyways, try these values for _ang:
-30,-20,-10,0,10,20,30
_dist is the distance from the tank, i.e. how far to fling the shells from their launch pods.