A fellow at the range yesterday showed me a fired 16 gauge reload with a punctured primer. The reload was a 1 ounce, Remington hull, with a W 209 primer. The gun was a near new CZ sxs. The primer was flush with the base.

What is the probable cause? How dangerous is this condition?

I have seen this happen in various shells from time to time, and in various guns sometimes with factory loads. The CZ is the newest gun I've seen do this.