General thought is that .025 is reasonable, .020 is where you start to take notice and below .020 proceed with caution. I shoot guns with .020 barrels. Where it is thin is as important and how thin it is. 20" from the breech is not a big worry for me. I do use "sane" loads. 5,000 to 7,000 loads will do everything I need. I shoot a Pape 20 bore with .016" & .020" walls, but that is at 26 & 27" from the breech, on a set of 30" barrels. In that gun, I have shot several thousand 7,200 psi loads. I would not shoot factory shells because I have no clue what pressure they might be. Reload and stay safe. I am more worried about recoil and stock damage than barrel wall thickness and pressure. Keep the pressure moderate and shot charge light and you will do fine.
