I purchased my GP-100 .357 several years ago and have used it for IDPA Revolver division using my cast bullets in the main. Mu cylinders were so tight 3 would not let a .356 bullet pass through them and three would not let a .357 bullet through. I just had all six reamed out to .358. I had my forcing cone cut to 11 degrees. Since doing this I have yet to have any leading shooting my RCBS 158 gr RN bullets sized .358 or any other lead bullet iteration sized .358.
In the process of dealing with my new GP-100 10MM. It has leading just past the forcing cone. I intend to shoot lead through it so it will spend time with my gunsmith to have the cylinders checked out and teh forcing cone cut to 11 degrees.
Take Care
I know all guns can be different but if you are looking for best accuracy and no leading I think the quick fix is what I described above. I doubt your cylinders are greater than .358 at the throat.