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I am no expert but maby it has something to do with makeing a better gas seal.
I think a "better gas seal" was desirable at the front of the chamber in the area of the cone but not at the breech area. Here is a picture of a Grade-6 T/A Parker Bros. hammer gun I photographed extensively for an article I have been working on. It is obvious that relieving the gas pressure at the breech faces was the desired result of this machining.
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