slower burning powders build higher pressures. This results in higher velocities causing the fouling.
I have had good success with HS-6, with 40 caliber and 165 grain bullet.
I also like HP-38 which is the same as W-231. I did some light loads for 38 Special and really like it.
The X-treme bullets I treat as a full jacketed bullet.
Using the data below I would suggest you do about about 10 rounds with 5.2 grains of powder, 10 rounds of 5.3 of powder and see which you like.
Just make sure your gun runs right. Once your happy with the load make a big batch.
Be sure to have the load recorded. 3.5 card, spreadsheet, just be sure to follow it expictly once you have a load you like.
load data from hodgdon
155 GR. bullet
Winchester 231 or HP-38
5.0 937 22,900 PSI
6.0 1103 33,000 PSI