Many English guns have barrels that thin and they pass proof.

I have a J & W Tolley with Damascus barrels proofed with one barrel at .024 near the muzzle. I have no hesitation to shoot it with my low pressure reloads or RST ammunition. In fact I've shot several thousand rounds through it.

The British have no problem down to .020 provided that it's near the muzzle.

I would have no problem with 2.5" shells in a gun with .022 barrels provided the thin area is near the muzzle.