I realize this is an old thread....I have spent the last 45 years selling doubles and servicing them. Have a competent gunsmith that knows early doubles especially, Ithaca's, as they have their own quirks. The Flues is not a tremendously strong action, better than its predecessors but it will not take the pounding of high pressure modern loads. That said, RST, www.RSTshells.com
makes low pressure 2 1/2" 16 bore loads as light as 3/4 or 7/8 ounce. These are black powder pressure loads and are safe in mechanically sound firearms, i.e. barrels tight on the face, solder joints sound, no deep pits or corrosion, free of dents or bulges in the barrels, etc. Before it gets shot it should be checked carefully by an educated 'smith. These were notorious for being shot for years with 2 3/4" loads because they fit.. and the result usually shows up at the junction of the standing breech and water table as a slight crack or BIG crack. As well as being very loose. Just because it has fluid steel barrels doesn't make it sound. Even if someone has re-chambered it for 2 3/4" doesn't make it safe for the longer cartridge. It was designed for lower pressure loads. One of the things I look for is cracked or broken and repaired butt stocks on these guns. The frame acts as a splitting wedge when fired with heavy (loads not intended for these firearms)loads they are often repaired with a cross pin or screw to pull the stock cheeks back tight to the frame. I hope it gets thoroughly vetted before it's fired.