I was playing around today with some calculators
with the twist rate of a Handi set to 18.75", a 250gr bullet will stabilize all the way down below 670fps depending on Pressure/Temp.
I recently got my hands on 1k .358 245grs bullets. cast fairly hard, but have a flat point.
I'm going to play with these as per the OP in my 357's, IF it ever warms up!