It has been proposed that the failure initiation point was the dark divot shown here, with failure of the braze joint + the now unsupported thin wall +/- a flaw (?inclusion) at that point; with the gasses being vented superiorly



After wiping with Shooter’s Choice, without use of an abrasive



Courtesy of Dewey Vicknair



The shell used in the burst chamber. Clearly the Cheddite hulls have a separate plastic base wad, part of which is missing. The brass base has fractured and a section of the hull is also missing.



Examination of the 7 empty shells used immediately prior to the burst (saved for reloading) shows the base wads are in place and complete, and each has a factory primer. 8 unfired factory shells have the same primer.

The burst shell IS A RELOAD with what is likely a Cheddite Clerinox 209 primer. The primer has been displaced out of the pocket, is bulging outward, and the (partial) base of the shell has a distinct extractor indentation.

Clerinox primer, burst shell, factory Cheddite/Herters right



Fractured and flattened rim



Extractor imprint



Major Sir Gerald Burrard The Modern Shotgun, Volume 3, The Gun and The Cartridge, “The Diagnosis of a Burst”, 1948
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.

Ballistic testing of the unfired factory shells, while of interest, is not relevant.
FYI:
Downrange Mfr. only tests 12g shells
Precision Reloading will not test factory shells; information regarding shipping (NOT by USPS) is here
https://www.precisionreloading.com/docs/uploads/BALLISTIC%20TESTING%20INFORMATION-2.pdf
Tom Armbrust will test 16g loads
1108 W. May Ave, McHenry, IL 60051
Note new (cell) phone: 815-451-6649


Measurements

The left chamber is 2 9/16” measuring .750” at the breech to .738” at the end of the chamber. Superficial tools marks are present in both chambers.
The left forcing cone is 9/16”; right could not be measured but visually appears the same.
Both bores at 9” are .650”.

A 1907 Hunter Arms engineering drawing specifies 16g chambers as .745” tapering to .732” with a 1/2" forcing cone to a bore of .650”.

Impression: Slight disparity in numbers likely insignificant. No evidence of modification to chamber or bore.

Wall thickness

End of the chamber L .096”; R could be measured and .105”
Forcing cone L .112”; R .126”
9” from breech L .046”; R .042”
9” from muzzle L and R .032”
MWT was several inches in mid barrel and both .028”

Impression: adequate wall thickness

SUMMARY

1. There is no evidence of chamber, cone, or bore modification; and wall thicknesses are compatible with other measured 16g Regular frame Smith guns.
2. The burst shell was a reload, and shows evidence of over-pressure.
3. There is visual evidence of failure of the braze joint, and a suggestion of a defect in the barrel wall which served as the failure initiation point.

Unanswered questions:

Would the chamber have burst without the over-pressure shell?

Is the top rib extension wedge brazed to the thin medial barrel wall a design defect, or was this a manufacturing error?

Did the use of 2 3/4" shells in a 2 9/16" chamber add to the over-pressure? The once fired Cheddite hulls are a full 2 3/4". Sherman Bell's study of 2 3/4" shells in 2 1/2" chambers showed a rise in pressure from 228 psi to 1216 psi compared to 2 3/4" chamber with a 1" forcing cone.


I plan on asking an engineer at METL for an opinion regarding the failure of the braze joint.



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