I am fiddling with a project to get my 221 Fireball to shoot subsonic loads with a good sound-suppressor.
So far it is working with the recommended 2.4 grains of shotgun powder (MS 200)
As an aside, what does shotgun powder do differently with higher charges?
(I am not planning to experiment!)

I would suggest you logon to WWW.Predatorextreme.com, a magazine dedicated to your subject. A very recent article addressed this topic.