For clarification, and lest the unknowing review this thread and make an unfortunate mistake, the wall thickness of both those barrels has been carefully measured.
I would suggest the first question before providing load advice should be "What is the wall thickness at the end of the chamber, the forcing cone, 9" from the breech, 9" from the muzzle, and the minimal wall thickness and where?"
Bruce has posted a 1900 Parker hang tag several times which states that the 12g 2 5/8” chambered gun was patterned at 40 yds. in a 30” circle using 2 3/4” shells with 1 1/8 oz. No. 7 chilled shot and 40 grains (
3 1/4 Dram) of DuPont Bulk Smokeless powder.
1920s tags listed both 1 1/8 oz. and 1 1/4 oz., still with DuPont Bulk Smokeless.
More information about the shells our vintage guns were designed to shoot here
https://docs.google.com/a/damascuskn...UOZEFU/preview