The original BP loading for .38 S&W: 14.125 gr Black Powder, 146.3 grain bullet. The 10 shot avg. velocity was 635 ft/sec @25' with a 5" barrel. Data from the 1890 Army revolver trials. Obviously, MV would be a little higher.

For awhile, I used 3.5 grains of Unique, but felt it was a little too much. I believe I was getting somewhat over 700 fps with that, so I dropped back to 3.0 grains. When shooting a .38 DA, one cannot expect one-hole groups, even at about 50', which is my usual firing distance. So I don't worry too much about minimizing SD. Just getting in the black is OK with me. My .38 DA has a 6" barrel.