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Other possible problems:
Cases are sized a bit on the short side, the fired in a chamber a bit on the long side. Initial pressure in the case sidewalls grips the sides of the chamber and the primer backs out a bit.
Then pressure builds more and the case either slips back a bit (okay IMO if it's only a small distance) or stretches back (bad if the distance is long). The backed-out primer then gets smashed into the outer corners of the primer pocket.
Run a patched rod down the bore to near the throat and fire a primed case (check, double-check, and re-check that it has no powder!). The distance the primer backs out can give you a clue.
The other potential cause that comes to mind is that the load was developed for a case with greater internal capacity than the ones you're using. This is not as much of a problem with .223/5.56 as it is in .308/7.62, but it *can* be a factor.
If your velocities are higher than advertised (or if you're shooting a shorter barrel than the load data used and are getting the same velocities), your load may not in fact be safe.
What primer are you using? Some "flow" more at top pressure than others...
Hope this helps.
Cheapo.
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