I think there might be a few options, firstly I would be seeking another opinion on the thickness. Just to be doubly sure before you decommission or sleeve/sub gauge tube the gun.
Second if it is that thin, see what your options are for sub gauge tubes. Obviously most of the pressure happens at the breech,and when the shot/wad of the sub gauge exits the tube, the pressure drop would be significant due to the space between the wad and the original bore, allowing propellant gas to escape? it may be that you can get away with shooting it with sub gauge. I am no gun smith but just trying to apply logic to keep your gun shooting. If what I have said does not ring true to anyone here please feel free to shoot it down in flames.

What is it about these old guns that draws us in?
www.firearms4sale.com.au