In The Parker Story on pages 96, 130, and 141 the history of the bolt and corresponding bite surface is discussed. In 1905 the mortised and pinned (replaceable wear plate) were patented and put into use in all Parker shotguns and even earlier guns which were sent in for any kind of service were fitted with the 1905 design.
I didn't find any reference to the angle but I seem to remember someone, maybe Dave Suponski (a machinist by profession), saying the angle was something like 3.5 or 4 degrees.
Perfectly flat mating surfaces wouldn't cause the bolt to become stuck but an angle of less than the proscribed degree could cause jamming.
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"I'm a Setter man.
Not because I think they're better than the other breeds,
but because I'm a romantic - stuck on tradition - and to me, a Setter just "belongs" in the grouse picture."
George King, "That's Ruff", 2010 - a timeless classic.
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