Thank you Karl, and that matters.
You have a wonderful lightweight gun, and I would use shells dimensionally and ballistically similar to those for which Parker Bros. designed the gun. I don't think we are any smarter than they were, now 100 years later
In the 1920s that would have been a 2 9/16" shell, 1 oz. shot, and 2 1/2 Dram Equivalent of Bulk Smokeless at 1165 fps at about 7000 psi, or Dense Smokeless at about 8000 psi.
For targets, 3/4 oz. are delightful.