I shoot a century old 16ga German sidelock which has an area of .020" thickness about 6 inches short of the muzzle. RST 7/8oz 2 1/2" shells are pretty steady diet for it. If it were that thin within first half of the barrel I would not. But there are more factors to consider. Look at a pressure curve for typical shotgun shells and you will see that the pressures really drop off dramatically once a few inches beyond the chamber area. Some of the newer powders giving high velocity loads in latest shells extend that high pressure area further done the bore to attain those velocities. So they are definitely not for older thinner barrels.
