I have a .38-55 that has been troubling getting it to shoot well to say the least. Big groups, I'd mess with it a bit, set it aside and come back to it.
Kind of figured after reading that the expander plug was the problem, so I splurged on a set of the Cowboy dies and shot a couple of groups today I loaded on the new dies. Made a world of difference, shrunk the groups I was getting by more than half. I can't remember ever having such a simple change made that dramatic a difference.
Call me a fan of these cowboy dies now. Next step is to try a set in .45-70 I think.