Check and see which cylinder opening is consistently missing. Most likely, it was bored too deep. It might be possible to have a smith re-fill the edge of that chamber cut some and then re-run the chamber reamer. If it's inconsistent in terms of chamber, it's most likely the ammunition not being sized consistently or length trimmed consistently anyway. Is this reloaded ammunition or factory and if factory what brands? Have you tried a couple different brands? Magtech tends to have thin brass and the edge might be slipping off the chamber slightly too. Or the primers might be a harder type. I'm betting it's ammunition, but there's too many variables if it shoots .357 fine.
