A private (non-governmental) proof testing facility was in operation in the U.S. at least by 1894
Forest & Stream, November 24, 1894 “Gas Pressure or Bursting Strain”
https://books.google.com/books?id=19kwAQAAMAAJ&pg=PA459&lpg
"The proof-house charge of the American Testing Institution for a 12-bore gun develops a gas pressure of over 18,000 lbs. to the square inch."
It is believed however that Hunter Arms proved in-house and the statement “All L.C. Smith guns are guaranteed to shoot any Nitro Powder made” appeared in advertisements as early as 1894.
The 1899 Hunter Arms catalog listing for the No. 0 grade states that the barrels are “...good quality Damascus...made to stand the rough, hard usage to which a cheap gun is subjected. The barrels will stand Nitro Powder, as will any other gun barrel sent from our factory, and are so guaranteed.”
The L.C. Smith listing in the 1908 Sears, Roebuck & Co. Catalogue No. 117 included the NOTICE “All our guns are tested with heavy loads and cannot burst except by carelessness, obstruction in the barrel or improper home loaded shells with nitro or dense powder.”
The stylized PM and HACo in a square over NP (Nitro Proof) first appeared on the barrel flats about 1922.
Hunter Arms Co. Proof Pressures, 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.
Pressures were 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” 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 these numbers have been switched as the 16g pressure should be higher.)
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 + 10 - 14% or about 16,000 psi.
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