In my old M49 steel J frame at 21 oz......................

3.4grs of Bullseye with that Speer 158gr LHP bullet at 1.48 oal
crossed my chrony at 670fps.
My full load did 752fps but was no where near my ...........

Green Dot, Unique or Blue Dot +P loads from 822 to 851fps.
948fps is possible, but why...?? !!

Factory loads in a 2" are from 780 to 850fps maybe.
Stay safe.