I think clays and red dot are too fast. SR7625 would be the fastest I'd try - and would try longshot if I had some.

My own work was all with round balls and SR4756 and SR7625 powders. I started with cup wads (gas check off a WWAA12) cup-up under the undersized ball and the top load I could find for a 1 1/8ounce charge of shot with that powder. I used fiber wads to get the correct height for folded crimps. I then increased the charge to obtain 1,550fps and felt I'd hit close to what the original 7 dram BP (191gr.) load ran - ie: 1,550fps. It worked well in my double shooting like a rifle to 50yards and like an exceptional smoothbore to 10 meters. I was able to hold offhand on a 12" steel plate at that range, shooting offhand and alternating barrels, right, left, reload, right, left. I also mounted a scope on the shotgun for load testing - no rifling.
See Ed huble's thrad on 12 bore from hell. Start on page 1 if not familiar with the thread and you will find good 'data' in there somewhere for using something like H4227- slow pistol, fast rifle powder. Wads, crimps and primers are very important for good ignition with the slow powders.