In an AR-15, with a floating firing pin, what you describe is normal when using CCI400 primers, even at 223 pressures.
As you creep up in pressure, the first thing that happens is that the primer strike dimple starts to turn inside out, returning almost to its initial position. Then, the dimple turns inside-out, producing a nipple on the primer. Just a little more, and the nipple fractures around its base, freeing a small piece of primer to fall into your action.
If this is your problem, then a switch to CCI450 or other thick cup primers will cure it.
I believe that pressures can cause cratering even with an AR, I was testing some loads the other day and when I went home I checked the cases and noticed that all of the primers had slight cratering and most had case head swipe, backed off three tenths of a grain and shot them, cratering and case head swipe disappeared. I was using Cci 400 primers, it is possible that with another primer things might be different but with both cratering and CHE evidence would point to slightly excessive pressures. If you send me a PM I can give you more details regarding the load.
