Did you say ".223" chamber?
The .223 and 5.56 are not exactly the same size. If I remember correctly, .223 is a bit tighter chamber ( a bit shorter in length ) compare to the NATO 5.56 chamber...
Check with some vendor web sites, they do talk about this.
If you have .223 chamber and are firing 5.56 NATO, you can get excessive pressure. Check the back of your case, is the primer area kinda flaten a bit? If it is the case, the brass is expending too much under the higher pressure and wiged in the chamber...
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