HS-6 for 38 Special
243winxb's results are typical.
I stopped loading HS-6 with both 38 and 44 Special. There just isn't enough pressure there.
I suppose if you went with a jacketed 158 and pumped it up real hot (maybe beyond +P??), it would eventually clean up. I won't bother.
HS-6 is a low-energy propellant that needs to develop significant pressure in order to run good, clean, and consistent.
On the good side, once you find the sweet spot, it's a very consistent running powder. And that "sweet spot" is easiest to find when it's pushing heavy bullets.