Yep, either a short/tight throat, or just the chamber is too tight for cast bullets. My LW barrel was that way; it wouldn't take .401" cast bullets with most brass. I lapped the chamber using a couple pieces of fired brass, no issues now. You only need to remove a couple thousandths, so it's an easy job.
I firelapped mine too, to open up the throat a bit. The LW rifling needs some smoothing anyway. CM, being a sixgun guy, I assume you're familiar with firelapping, using your cast bullets? You'll have to open up the chamber first, of course.