Martin- I'd go with the biggest ball mould that is easily obtained, yet doesn't touch the bore. Leading is a pain to removed.

If you don't want to, or are not allowed to cut the barrels shorter, then the .690" is the one to get. Lee makes the mould, so that's about $20.00 US. Tanner moulds in that size run around $30.00 and you'll need handles too.

I mounted a scope on my double 12, just for load checking so I'd not have an aiming error. After that, I dovetailed in a nice rear sight form Track of the Wolf to go with the dovetailed bead at the front.

Say - is there a trap range close by? The gas checks cut off trap wads, make good ball centering devices. Range pickup makes them pretty cheap. On the trip out the barrel, they hold the ball exactly in the middle of the tube, so there's no bouncing side to side, top to bottom.