A 1922 vintage Parker Bros. Trojan 20-gauge most assuredly left Meriden with 2 3/8 inch chambers intended for 2 1/2 inch shells. Hard to say what may or may not have been done to it over the past 97 years though. Good to have it checked out by a competent double gun smith with the proper tools. The heaviest loads most of our North American ammunition manufacturers offered in the 2 1/2 inch 20-gauge case was 2 1/4 drams of bulk smokeless powder, or 18 grains of dense smokeless powder such as Infallible or Ballistite, pushing 7/8 ounce of shot --
Remington Arms-Union Metallic Cartridge Co. ARROW 20-gauge.jpg
but lighter loads were quite commonly used --
Nitro Club 20-gauge 01.jpg
In 1922, Western Cartridge Co. introduced the heavier, high velocity, 1 ounce, 20-gauge load, using progressive burning smokeless powder, which they called Super-X, and it was put up in their 2 3/4 inch FIELD shell.
Early Super-X 20-gauge box, Super Excellant.jpg
While most of our ammunition manufacturers followed suit and put their progressive burning powder, high velocity, 1 ounce 20-gauge shells up in a 2 3/4 inch case, Winchester with a boatload of their 20-gauge Model 12s out there, made for 2 1/2 inch shells, managed to stuff the 1 ounce high velocity load in a 2 1/2 inch case.
Super-Speed 2 1-2 inch 20-gauge.jpg
but also offered it in a 2 3/4 inch case --
Super-Speed 2 3-4 inch 20-gauge.jpg