I have a 357 barrel in a G22. Works well. Mine happens to be more accurate than any other Glock I use. Does a G20 have a heavier recoil spting than a 40 cal? might need a lighter recoil guide/spring. A $10 part.
I just re-size 40 cal brass. I don't size down to die hits shall holder, so as to give less headspace on case.
The 124 plated FP works well, as does the 130gr lead FP bullet, sized 357.