Thanks, now that I know it ain't supposed to do dat, I played with it a little bit more. I had some Norinco ammo that was giving me the problem, so I tried some Q3131A. It went through fine. So I went back to the Norinco, and after initial testing, the rounds that were jamming and those that weren't, were repeatable.
It's starting to look like it may be more related to slightly out of spec ammo, not just the tight chamber. I think I'll play with it some more(!) and confirm this.