Re: .223 Headspace problem?
I don't have a definitive solution for you but here are some ideas to check. I don't think that it is a headspace problem because your Blackhills LC brass shows no signs, right?
I would recommed getting some good brass, Lapua or Norma, load them and see if you have the same problem. Sounds like the primer pockets are not to spec on the winchester brass or the webbing in the case head is too thin causing the primer to push back after firing.
I would also check the fired cases against the unfired cases and see if the shoulder angle is different, if the case has streched more than normal. If so, then that would be a better indicator about a headspace issue.
I hope that this helps.