I checked some Unique loads at 5.5 gr with a 158 gr LSWC (by LAH/Creeker) and got 950-1000 fps with 4" barrels. Some of the Unique I used was made in the 1940s and some was from 2005. There was about 25-30 fps difference between the two lots.

Using the same bullet and gun (4" M66-2), 9.0 gr of SR 4756 produces 1200 fps and 13.5 gr of 2400 produces 1260 fps. Switching to a 358156, 9.0 gr of SR 4756 drops to 1180 fps.