OWD, the shell did come before the gun. I can't tell you the reason behind this particular brain child of Olin's, but he did have the idea for the load before contacting Lou Smith at Ithaca to build the gun to handle it. I've always found it interesting, given the Model 21's reputation for strength, that Olin didn't use the 21 as a platform. But Ithaca's NID was probably a good choice.
Re the original payload for the 10ga mag . . . as of 1940, both Super-X and Remington Nitro Express shells were available with 2 ounces of shot.
