Thank you, I stand corrected. I always thought that the proof houses had a minimum standard of what they would accept as far as wall thickness. I confused that with what the gun trade informally holds as a minimum standard.

The U.S. has no proof house, as opposed to the U.K.

Caveat Emptor

John is correct, which is not to say the guns were not proved.

Parker Brothers 1893 Catalogue
“Our guns are bored on the latest improved system for shooting Nitros, or Smokeless Powder, and all our guns are tested with some one of the most approved makes, and a tag accompanies each gun, giving the results of such a (pattern) test.”
(No records exist regarding the specific proof load in that time period.)

“A Trip Thru Parker Bros.”, 1923. Courtesy of Jeff Kuss

http://pic20.picturetrail.com:80/VOL.../412230865.jpg
http://pic20.picturetrail.com:80/VOL.../412230864.jpg

A Parker Service and Proof Load table was published in the 1930s and reproduced in the 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 measured using LUP and modern transducer values would be 10-14% higher, or more than 17,500 psi.

LTC Calvin Goddard writing in “Army Ordnance” in 1934, reported that Parker followed the SAAMI standards then in force: 13,700 psi proof, 9500 psi service for 2 5/8" chamber; 15,900 psi proof, 10,500 psi service for 2 3/4" chamber (by LUP) + 10-14% for modern transducer measurement.

Interesting article. Two questions come to mind.

1. If the barrels are not properly regulated upon POI testing how did they change the regulation on a finished set of barrels?
2. The article states that after proof each barrel is stamped on the flats. What does that mark look like?

Here's an example on a late Remington Parker but that "OVERLOAD PROVED" stamp was used on Parkers in the later Meriden days.


.

Looks like the article Drew provided above is dated 1923.

It is well to keep in mind that the "proof safety" of any gun barrel is only valid at the factory, and for as long as the subject barrel remains unaltered.

Within the decades that many of double guns we admire have existed, they have stood liable to a panoply of factors that can negate proof. Parker, British, or otherwise. Most of these factors involve a host of bodgers who may have attacked the barrel with various implements over the years. To shorten, open, hone, lengthen chambers or otherwise alter the barrel to a less than safe condition. Especially with the use of certain ammunition.

Thus, most if not all classic doubles should be assessed by a professional and competent double gunsmith before any verdict is rendered as to their safety and suitability for shooting today.