"I don't think we can learn anything from this exercise."
Few more insightful words have been typed
HOWEVER, having participated in a number of barrel bulge/burst and shotgun blow-up discussions I'll babble anyway
I believe a 2 part barrel burst failure analysis and metallurgical study will appear in the
DGJ Fall issue.
Lots of information, and scary pictures here, along with some anonymous sniping
http://www.shotgunworld.com/bbs/view...p?f=5&t=366087
1. A Parker Service and Proof Load table was published in the
1930s and reproduced in
The Parker Story p. 515. 12g 2 3/4” shell Service Pressure is 10,500 psi. Definitive proof used 7.53 Drams (no doubt) black powder and 2 oz. shot with a pressure of 15,900 psi. The pressure was (likely) measured using LUP and modern transducer values would be 10-14% higher, or more than 17,500 psi. I am not aware of any definitive information as to turn-of-the-century Parker proof pressures.
2. Under the 1896 British Rules of Proof, 12g 2 1/2” and 2 5/8” chambers (bore .710-.740) maximum service load was of 3 1/4 Dram Eq. with 1 1/4 oz. shot.
https://books.google.com/books?id=in...AJ&pg=PA296&dq
Definitive Proof – 6 1/2 Drams Proof-House Black Powder with 1 2/3 oz. No. 6 shot = 10,100 psi
+ 10 - 14%
Supplementary Nitro Proof with 4 1/2 Drams of C&H No. 2 T.S. powder and 1 2/3 oz. shot = 16,400 psi
+ 10-14%
3. 16 bore British Service charge was 1 oz. 2 3/4 Dram Eq. = 38 gr. Bulk Nitro powder. Published pressures
+ 10-14%:
C&H No. 4 - 7,480; “Schultze” - 8,250 psi; “E.C.” - 8,960 psi
The U.S. 16g standard was 1 oz. with 2 1/2 Dr. Eq.; likely about 7500 psi with Bulk Powder by modern transducers
4. There are a number of bursting pressure formula; Burrard used the Alger Burst Formula:
Burst pressure = Ultimate tensile strength x 3(OD – ID) / OD + 2xID
There is also Lame, Boardman & Lame, American Standard, and 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.
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. I've discussed this issue with a mechanical and a metallurgical engineer, and there is essentially NO formula that can be used for shotgun barrels. Nor can the Hoop Stress Formula be reliably applied.
NOTE: every formula requires knowledge of the tensile strength of the metal AND the wall thickness.
Without that information any speculation is at best a guess.
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.
4. From Major Sir Gerald Burrard,
The Modern Shotgun, "Volume 3, The Gun and The Cartridge", 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…”
Odd to have obstructional bulges at the same place in both barrels, and Burrard also makes the point that "rib lifting" almost always occurs with ring bulges. which I do not see.
5. 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 12 gauge barrel shooting a 28 gram (slightly less than 1 ounce) load. Peak pressure occurred 22mm (.866”) past the leading edge of the obstruction.
6. This is a 1902 16g No. 0 L.C. Smith with chambers extended to 2 7/8" with wall thickness at the end of the chamber of .096". Likely the victim of 16g "short magnum" shells with 1 1/4 oz shot
Radiography thereof
7.
So what happened to these barrels?
My opinion,
which is of no greater validity than anyone else's, is that this was NOT obstruction but inadequate wall thickness from previous honing.
"Mirror bores" in any vintage double should be huge
red flags.
8. It might be relevant that some "low pressure" loads use powders with a slower burn rate, producing a lower peak pressure BUT slightly MORE pressure further down the barrel. Note the DuPont Oval curve below
Red Dot vs. PB (no longer available)
