Sir Gerald Burrard, The Modern Shotgun, Volume 3, The Gun and The Cartridge, The Diagnosis of a Burst 1948
"In the case of an obstructional burst the really essential evidence is the ring bulge. If there is a ring bulge, there must have been an obstruction; and the absence of a ring bulge is conclusive proof that there could not have been an obstruction."
"A pressure burst can only occur in the immediate neighbourhood of the chamber; and so if the burst occurred ahead of the chamber cone an excessive pressure can be ruled out. But if the burst occurred at the breech, and was the direct result of a high pressure, confirmatory evidence will be found in the appearance of the brass head of the cartridge which caused the accident. For it is utterly impossible for a very high pressure to be developed without it leaving its mark on the fired case."
Burrand identified an indentation of the extractor on the case head, enlargement of the case head, flattening or fracture of the rim, lifting of the primer from the pocket, flattening of the primer against the breech face, and deep striker indentation, especially in comparison to shells of the same batch, as evidence of excessive pressure.
There are several formula for bursting pressure, esp. Barlow's
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.
Barlows 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.
While working on an article regarding a chamber blow-out and the formal failure analysis, I discussed bursting pressure with a metallurgist and a mechanical engineer. Both were of the opinion that we do not have a formula for shotgun barrels (non-pressure cylinder). The various hoop stress formula are helpful but again do not predict bursting.
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.
I also found a study by the Royal Military College of Science, sponsored by the Birmingham Proof House and the British Association for Shooting and Conservation, showed that an obstruction by 2 fibre wads (total weight of 4 grams) was sufficient to bulge or burst a 12g barrel shooting a 28 gram (slightly less than 1 ounce) load. Peak pressure occurred 22mm (.866) past the leading edge of the obstruction.
