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I’m looking at an early (1872) Francis Brebner hammer gun that was reproofed about 10 years ago according to the proof marks. Barrels were reproofed at 70mm which leads me to assume the chambers were lengthened. Is there any way of knowing if the chambers are, in fact, original to the gun. I suspect the gun may have been made for the pigeon ring due to its embellishments but that may be wishful thinking. Did black powder proof ever account for chamber length. Bores are still within the original .729” proof at .736”. Thanks

Greener discussed the 1855 and 1875 Rules here
https://books.google.com/books?id=LAsAAAAAQAAJ&pg=PA310#v=onepage&q&f=false
and the Rules made no mention of chamber length; which was typically 2 1/2" but could be 2 3/4", 2 7/8" or 3"

What the Rules did specify was the standard service load, and a gun was provided with a hang tag or card in the gun case specifying the load for which the gun was intended.
The case label with a c. 1880 William Sumners, Liverpool 12b specified Curtis & Harvey No. 5 Black Powder:
“Light” - 2 3/4 Dram with 1 oz. shot = 1180 fps
“Medium” - 3 Dram with 1 1/8 oz. shot = 1200 fps
“Heavy” - 3 1/4 Dram with 1 1/4 oz. shot = 1220 fps
but no mention of cartridge length.

Gene probably posted this from 1896. Note the "Schultze" and "E.C." would be 42 and 45 grains



The Max. service load oz. was not part of the proof marks until 1896, well into the Nitro powder era.
Thereafter, 1 1/8 oz. was for 2 1/2" chambers and 1 1/4 oz. for 2 3/4" chambers.
Under the 1896 Rules 12g chambers of 3" or longer were indicated by 12 over LC in the diamond
In the 1904 revisions 12 over LC was longer than 3"

Makers did occasionally mark the case length, mostly for Pigeon guns; a pre-1896 Wm. Cashmore



Actual chamber length was not marked until post-1925.