If you're talking about the "top" as the mouth of the case, then it's probably ammo. If it's the old surplus stuff, then it's my vote that's the cause. Both of mine will do it with certain old, cheap, junk surplus ammo.
Headspace problems usually manifest themselves in cracks/splits at the case head because it stretches too much there and finally gives way.
Also roller delay guns like the HKs, MG42 and CZ-52 are pretty brutal on cases as the gun really doesn't lock. The rollers just delay what's going on, but it still goes on the whole time.
If you really want to be sure, buy a box of new stuff and run through it. I'd recommend a box of the Norinco. That stuff's a hoot! Big flame, alot of noise, and serious blast. It's especially cool at an indoor range.
Ross