The barrel failed because of the critical juncture of 3 factors:
1. An over-pressure load
2. A defective braze of the top rib extension wedge to barrel.
3. Inclusions (possibly a large inclusion) in the barrel wall.
The barrel did not fail because of low cycle metal fatigue.
Did the use of 2 3/4” shells in a 2 9/16” chamber add to the apparent reload 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 with a 7/16” forcing cone showed a rise in pressure from 228 psi to 1216 psi compared to 2 3/4” chambers with a 1” forcing cone.
Hunter Arms produced about 530,000 Smith sidelocks, and another about 80,000 Fulton boxlocks. If the top rib extension wedge brazed to the thin medial barrel wall was an intrinsic design flaw which created time-bombs, one would think a plague of blown barrels would be apparent by now. Clearly THIS 110 year old barrel had a manufacturing defect and inclusions (and likely a large inclusion) in the barrel tube, but would it have failed without an over-pressure load in a short chamber?
A study complimentary to the Birmingham Proof House Trial was published in in The Field June 6, 1891 by Horatio F. Phillips, a “staff experimenter”, comparing brazed and unbrazed Steel and Damascus barrels
“These experiments serve to show what a very large margin of strength there is in a good gun barrel, when ordinary charges are used. The (Damascus) barrels which gave way earliest...had withstood the strains of…about four times as great as the regulation proof; while the steel barrels (Siemens-Martin and English “Superior Barrel Steel”) were tested…with charges averaging nearly five times as much as the ordinary proof-charge.”
It would seem that this large margin of safety was what saved this gun until 2019. It is significant that there was no microscopic evidence of low cycle fatigue - the pressures to which the top rib extension wedge braze and barrel were subjected was below the yield strength of the steel. The failure was initiated at an area with inclusions and a contaminated non-fused braze joint - not low cycle ductile fatigue progressing to plastic deformation (stretching) and terminal cleavage.