From the 1890s onward, our North American ammunition companies offered paper 20-gauge NPEs and loaded shells in four lengths -- 2 1/2, 2 3/4, 2 7/8 and 3-inch. The "standard" 20-gauge shell was 2 1/2 inch and carried a maximum load of 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. Back in the day, our ammo companies didn't include the shell length on the box labels of "standard" length shells.
Remington Arms-Union Metallic Cartridge Co. ARROW 20-gauge.jpg
The 2 3/4 inch, and longer, shells could be had with a slightly hotter load of 2 1/2 drams of bulk smokeless powder or 20-grains of dense smokeless powder such as Infallible or Ballistite pushing the same 7/8 ounce of shot. The perceived advantage of the longer 2 7/8 and 3 inch shells was more/better wadding.
20-gauge 3-inch Rem-UMC Arrow.jpg