DON"T USE...
valve grindibg compound, either try a new brass bore brush or slot the end of brass / Aluminum ros and insert a piece of the WHITE 3M stoch-brite pad, (from Body shop) and slowly spin that inside the chambers, Use plenty of WD40 as lube and to clean the residue.
Have you shot a lot of .38 Spl's in a .357MAg chamber, then you might have a debrie ring and the longer .357 are habging up.
I have a friens that shot indoor .22 maatches with S&W M17, with .22 shorts,
when he use .22LR, he had hand-ups.