The chamber cleaning brush arrived, I put it to work, and also gave everything else on the gun a pretty thorough cleaning and lubing, and that seems to have done the trick. A session of about 130 rounds - a variety of hd type rounds as well as fmj and no repeat of the issue. It did get filthy, and failed to hold open a couple of times towards the end, but I can live with that. As Wiskeyhorse says you gotta clean these little buggers, and I doubt I'll put that many rounds through it again in one sitting.
Thanks Wiskeyhorse, and everyone.