That small bit if metal... was it ring shaped with three small dimples around its outside? It sounds like your cartridge blew its neck off. I've never heard of it happening on a CZ 52, and I've never seen it either with my own eyes, but my dad mentioned that that happened to a rifle he had to repair when he was in the Marines.
If that bit of BB sized metal was round and had little tabs off it, it may be the top of the primer cup from inside the shell. I've never heard of these blowing themselves out either. But, there's a first time for everything, I suppose. If it happens again, check every empty shell you have lying around to see what the primer housing looks like inside the shell. (Yeah, it's a small hole and its awfully dark in there, but nobody will mind if you conduct a little post-mortem autopsy on the empty shell. Cut it open and have a look in the name of science.)
Scott