From the Griffin & Howe site:
http://www.griffinhowe.com/abbreviations.cfm
"A rule of thumb states that the minimum barrel wall thickness should be .020" in a 12 gauge gun." per the British Gun Trade Association

From the PGCA site:
http://www.parkergun.org/forums/forum1/595.html
We who shoot Damascus barreled guns look for a minimum of .025 wall thickness...and .080 at the end of the chambers.
Customary measurement is 9" from muzzle and breech.

UK "Working Standards for Re-proof" minimum wall thickness measured 18" from the barrel breech from Double Gun Classics p. 56, Vol. 1, No. 4, Jan-Feb, 2006':

2 1/2" 12g- .028
2 3/4" 12g- .032
Re-proof recomended minimum- .024

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