http://parkerguns.org/pages/faq/BarrelThickness.htm
I just noticed that
the barrel thickness image is missing the source -
The Field March 7, 1891 Vol 77:325
And some more sources that could be added to the FAQ
Christopher Austyn
Modern Sporting Guns
"There is no legal minimum thickness for a shotgun barrel, although the British Gun Trade Association recommends 20 thousandths of an inch as a general minimum."
Courtesy of Hugh Lomas for English 12 bore Game guns. Small gauge guns may require additional thickness.
Pre 1925 Proofs:
Chamber Immediately prior to Forcing Cone - .105"
9" from Breech - .045"
4-8" back from muzzle - .025"
Post 1925 proofs:
.100"
.040"
.025-.020"minimum
Michael Lynch, the chief inspector of The Birmingham Proof House, states that they inform customers only if the MWT is below .020" after having passed proof.
George Gibbs Ltd will sell secondhand shotguns with a minimum of .004" of proof size remaining and a minimum of .025" of barrel wall thickness at 9" from the muzzle.
http://gibbsgunmakers.com/Notes%20on...20Shotguns.htm
Courtesy of Austin Hogan
Analysis of 22 twelve gauge Parker barrel sets show the average minimum barrel thickness at 24 inches is .031 inches. Minimum thickness at 18 inches was about .050 inches.
Eccentricity may cause portions of the barrel to be thinner.