Not sure if you could find a load that keep the powder charge the same from 12 to 20. If you do the velocity will be different i'd think. I also think you'd be better suited for skeet shooting if the velocity stay the same so your swing is not effected. Just my thoughts. You need a few good manuals like the 5TH edition of lyman shotshell and study the 3/4oz loadings for each. There is a lot more to loading shot shells than metallic reloading.