By and large Parker Bros. had the policy of holding chambers 1/8 inch shorter than the intended shell. This remained even into the Remington years, see pages 164 to 169 of the book
The Parker Story for the Remington era specification sheets. Semantics really come into play on this. If a customer requested that his/her gun be chambered for 3-inch shells, the gun would be delivered with 2 7/8 inch chambers. If the customer ordered "3-inch chambers" supposedly that is what the customer would get.
Long shells have been around just about as long as cartridge shotguns have been around. In my 1903 UMC catalogue there were 12-gauge paper shells 2 5/8, 2 3/4, 2 7/8, 3 and 3 1/4 inch. The 16-gauge was available in 2 9/16, 2 3/4, 2 7/8, and 3 inch. The 20-gauge was offered in 2 1/2, 2 3/4, 2 7/8, and 3 inch. In those days the "standard" 2 5/8 inch 12-gauge, 2 9/16 inch 16-gauge and 2 1/2 inch 20-gauge shells carried a slightly milder maximum load than the 2 3/4 inch and longer shells. The advantage of the 2 7/8, 3 and 3 1/4 inch shells was more and better wadding for a better gas seal which many serious Pigeon shooters thought to be an advantage. The heaviest loads offered in the 2 1/2 inch 20-gauge shells were 2 1/4 drams of bulk smokeless powder pushing 7/8 ounce of shot, or 18 grains of Ballistite or Infallible dense smokeless powder pushing the same 7/8 ounce of shot. In the 2 3/4 inch 20-gauge case, one could get 2 1/2 drams bulk smokeless powder or 20 grains of Infallible or Ballistite dense smokeless powder, but still pushing 7/8 ounce of shot. The 3-inch 20-gauge shells for the famous Widgeon Duck Club Parker Bros. guns and the J. Stevens A & T Co. No. 200 pump guns carried these 7/8 ounce loads, but more wadding.
These are the shells a Parker Bros. 20-gauge, chambered for 3-inch shells, during at least the first quarter of the 20th Century would have been made for --
Same load in a 2 7/8 inch shell --
And, the same load from Peters Cartridge Co. --
