I've been using lately lots of Hodgdon HS-6 but it is one dirty powder.
Today, after 100 round of drills with my 637, the cylinder became hard to turn. It was only noticeable when the cylinder was open and did not affect function.
It turned out to be carbon residue built up in the cylinder's front bearing where the yoke goes into the cylinder. It is apparent that normal combustion product leakeage around the forcing cone gap was depositing this in the bearing (which is right under the gap). A good scrubbing with CLP and all was right again in S&W-land.
But I would like some recommendations on cleaner burning powders for 38 Special loaded with 158 gr hard cast lead bullets at 850 to 900 fps. My local suppliers have a good selection of IMR, Hodgdon, and Alliant products.