6.8 grains under a 158 lead bullet in .357 Mag brass was the max load Alliant listed about 10-15 years ago. I loaded up a bunch for use in IDPA ESR and they ran about 1100 fps from my 4" 681.
Six grains of Unique under a 170 LSWC in .38 brass runs about 1070 fps from my 4" M28. 6.5 would put it well over 1100 fps.