I've owned a 12b double rifle, but never a Greener SS.

Nevertheless, I can tell you the best choice will be to go no longer than 2.75", and probably 2.5".

Round balls will be your best choice if the barrel has slow enough twist. In fact, if I were doing this project I would set it up for round balls from the inception. What is your twist rate?

Also, what is your groove diameter? That will dictate whether you should use brass cases or paper cases with a round ball. If it's a standard 12b (.735" grooves or thereabouts), use of brass is not going to be an option. A ball to grip the inside of a brass case will be too large.

Many questions. It all has to fit.

Curl