The revolver has a barrel/cylinder gap, and that produces some extra sound same as a tiny barrel port would do. I think you'll find a fast powder like Clays or N310 will let you make the least total gas volume and still get the bullet out of the tube. May help with the sound level.
I recall one woman bull's eye champion from the 60's used 3.2 grains of Bullseye with a 200 grain swaged lead SWC. I'm sure her goldcup's recoil springs were light.
In .45 Auto, the powder space in the case is so short you can load it down as far as still works. The starting loads published for it are not true minimums. They are just the minimum reduction they want people who are working up to maximum to use. It's not because you can't load lower.