In the early years of the Flues Model Ithaca built their smallbores very light -- 28-gauges as light as 4 3/4 pounds and 20-gauges as light as 5 1/4 pounds. These guns not only had thin barrels chambered for 2 1/2 inch shells, but also had thin frame sides which led to cracked frames when the North American Nimrod began shoving 2 3/4 inch Super-X 20-gauge shells in them!! I'd strongly recommend sticking to very light loads in 2 1/2 inch cases in an early Flues 20-gauge. RSTs or the little 3/4 ounce Lyalvales come to mind.