It was my hope that someone else would jump on this one, as it is a frequent topic, along with "Are my Damascus barrels safe to shoot?" At the risk of some personal injury lawyer deciding I have exposure as an 'expert', and naming me along with AJ Aubrey when somebody blows up their Sears best gun here goes
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 Proof recommended 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 recommended minimum- .024
(Note there is no
required minimum for re-proof)
The Hunter's Encyclopedia from the German proof house:
"Minimal wall thickness at end of chamber, regardless of length, for 12, 16 & 20 gauge guns should be 2.3mm (.0906") for 'ordinary good steel' or 2.1mm (.0827") if a 'Special Steel' was used. For the 24 & 28 gauges, due to their higher pressures, 2.4mm (.0945") was recommended. Minimal wall of .6mm (.0236") was recommended in the "forward third" of the barrel."
