We do not know however if "out" meant the gun would no longer lock up, if the stock disintegrated, or if the breech burst.
Ned Schwing, Winchester's Finest The Model 21, p. 22
"Instead of following the usual practice of utilizing case-hardened material for the frame, Winchester employed chrome molydenum alloy steel, the famous Winchester Proof Steel, having a tensile strength of 90 tons per square inch"
"When Winchester decided to build a double barrel side by side shotgun it wanted a set of barrels that were strong and accurate. The result was barrels forged in the Winchester plant from specially heat treated alloy steel having a tensile strength of 115,000 to 120,000 pounds per square inch and an elastic limit of approximately 105,000 pounds per square inch. This was more than twice the strength of the usual barrel steel."
"Model 21 barrels began as solid core blanks of special chrome molybdenum alloy developed by Winchester metallurgists working with the steel company´s metallurgists to devise a formula strictly for Winchester barrels.", p. 57.
The Winchester “Violent Proof” (VP) load was reported to be 7 1/2 (long) tons = about 24,000 psi by Burrard's conversion.
Hunter Arms Co. Pressure Curve, 6-10-1929 from the McCracken Research Library, Buffalo Bill Center of the West, and used with permission.
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/107
Proof Load with FFFg black powder. Shot presumed to be No. 6
Pressures measured by crushers (LUP - Lead Units Pressure) and modern piezoelectric transducer measurements would by 10 – 14% higher
10g 2 7/8” 9.5 Drams with 2.36 oz. shot = 16,000 psi
12g 3” 8.265 Drams with 2.187 oz. shot = 17,250 psi
12g 2 3/4” 7.53 Drams with 2 oz. shot = pressure not recorded
12g 2 3/4” 6.5 Drams with 1.687 oz. shot = 14,200 psi
16g 2 9/16” 6.5 Drams with 1.687 oz. shot = 13,750 psi
(It seems likely that the above 2 numbers have been switched.)
20g 2 3/4” 5.74 Drams with 1.5 oz. shot = 14,625 psi
.410 2 1/2” 2.377 Drams with .624 oz. = 15,625 psi
LTC Calvin Goddard writing in “Army Ordnance” in 1934, stated that Hunter Arms proof tested 12g 2 3/4” chamber barrels at 14,300 psi by LUP + 10 - 14% or about 16,000 psi.
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