I don't shoot .38 swc's at 50 yards, so I have nothing useful to contribute.
As an observation, it seems odd that both of your revolvers behave the same way. Excuse my ignorance, but why not just shoot the jacketed bullets that group well? Is SWC lead a requirement?
I have a lot of experience shooting big bore revolvers at 50 and 100 yards (when I was younger). Unfortunately none of it is relevant to your situation.
Lastly, make sure the brinell hardness of your bullets are proper for the chamber pressure of your loads. Good luck and good shooting.
Ray