The wirbel guns are zeroed for about 400yds if I remember from back when I was building the reticle. I built the white reticle for it from the german manual. I added the elevations to take the place of your spotter dialing them in. Unless you are inside of 1000, that default reticle won't do much good.
After AH3 goes live, I have an offline terrain that will be available where you can spawn a wirbel within 500yds of the drones. Then you can turn on the green lead computing gunsight and learn how to use the gunsight in the wirbel, osti, and M16. I wouldn't worry so much about shot patterns on the target with 4 ganged 20mm.