Keith and all:
WARNING: Tensile strength is only a part of the equation for estimating bursting pressure. If the barrel is made of Twist with a 50,000 psi tensile strength,
that does NOT mean that it will withstand a 10,000 psi load by a factor of 5. Barlow's bursting formula P=2 S t / D
P=Bursting pressure in psi.
S=Tensile strength of material in tube wall.
t=Wall thickness in inches.
D=Outside diameter in inches.
Barlow’s refers to a pipe capped at both ends with a static pressure (a pressure cylinder). Shotgun barrels are
not designed to be pressure vessels as one end is open and the pressure rises and falls quickly.
Burrard used the Alger Burst Formula
Burst pressure = Ultimate tensile strength x 3(OD – ID) / OD + 2xID
Lame Formula
Burst pressure psi = Ultimate Tensile strength x (OD squared – ID squared) / OD squared + ID squared
American Standard Formula
http://www.smt.sandvik.com/en-us/materia...rican-standard/ Wallace H. Coxe, in "Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" published by E.I. du Pont de Nemours & Co. in 1931 cites a study in which a fluid steel barrel was cut to 9” and capped, then a series of progressively increasing pressure loads fired. The barrel cap was blown off and barrel burst at
5,600 psi.
Don A. and Chuck H. can, and have, provided failure curves and hoop stress formulae. I've discussed this issue with a Mechanical Engineer and a Metallurgist, and basically
there is no bursting formula for shotgun barrels 
