I must continue to express a contrary opinion to that of my friends here on the Luger Forum.
The "stepped" chamber was patented in 1910 by Georg Luger (although it was in use in 9mm Parabellums earlier). It's purpose was as a seal, to keep the chamber free from unburned powder residue.
The 2-part chamber machining was in constant use until 1941, when it was replaced with a smooth chamber, presumbly to reduce the complexity, and therefore the time and cost, of manufacture.
The fluted firing pin was patented by Georg Luger in 1893, explicitly to keep a firing striker clear of grease and oil residue. The feature was added to P08 production by 1934. If you examine a Luger with one of these strikers which is in use as a shooting pistol, you will see how well it functions as a trap for grease and residue. This, along with the absence of any exhaust channel within the breechblock itself, mitigates against its purpose as a vent for overpressure. The small hole drilled in the bottom of many Finnish breechblocks, uncovered when the striker is pressed to the rear, would function very well as a pressure release.
I have examined damage such as that pictured by tenbears, above. In the examples I have seen, the fractures do not extend through to the relieved breechblock face, suggesting that escaping breech pressure is not the culprit. As a WAG, I wonder if it might rather be a result of impact fracture of the breechblock against the barrel/receiver face?
--Dwight
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