Per the link, the least wall thickness recommended by CIP for 'standard steel' at the end of the chamber is .079" and just past the forcing cones/4" from the breech .075".

Courtesy of Hugh Lomas for English 12 bore game guns with pre-1925 proof:
Chamber Immediately prior to Forcing Cone - .105"
Post 1925 proofs - .100"

Also see http://parkerguns.org/forums/showthread.php?t=1565&page=4

Last edited by Drew Hause; 09/23/13 02:44 PM.
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