The standard chambering for Ithaca 20-gauge guns was 2 1/2 inches throughout the Flues period, same for their 28-gauge guns. In the earlier Flues era Ithaca advertised very light weights for their smallbores -- 4 3/4 pound 28-gauges and 5 1/4 pound 20-gauges. These are the guns that have very thin frame sides that have been known to crack, and are also known for very thin barrels. These guns did not stand up well to the Super-X type 2 3/4 inch ammo that the North American nimrod began shoving in them around 1922. Over the years the advertised weights for Flues Model Ithacas crept up --
1912-13 20-gauge 5 1/4 to 5 1/2, 28-gauge 4 3/4 to 5 1/4
1914-15 20-gauge 5 1/4 to 5 1/2, 28-gauge 4 3/4 to 5 1/4
1919 20-gauge 5 1/2 to 6, 28-gauge 5 to 5 1/2
1925 20-gauge 6 to 6 1/2, 28-gauge 5 3/4 to 6 1/4