I have use w571 with 148gr lead in my old K19 6" that was quite accurate.

I stopped using IMR4227 in my J frame revolver after one outing
where the slow powders unburned powder flakes made it impossible
to turn or remove my cylinder from the housing, due to "Crud" build up.

I had to drive home and clean it and remove the cylinder screw
to pull the yolk out, in order to remove the cylinder !!

HS6 is the slowest powder that will now go into my snub nose loads.