Pressure peaks within or slightly past the chamber. Ten inches down the barrel everything is under 1,500 psi, at 5” you are just over 2,000 psi. So your highest pressures are within an inch of the chamber, dropping off very quickly there after.
I spent a lot of mental energy figuring out safe loads for all my doubles. I shoot low pressures to protect the stock not because I think my barrels are too weak to withstand the pressures. Ultra thin barrels are not going to be shot by me. Terms like proof and service pressure help define loads to use. Black powder loads give you a safe working pressures if you need more guidelines. Most are in the 5-6,000 psi range.
My 12 bore, low pressure loads, run from 5,000 to 7,000 psi. Anything in between works well and I feel safe shooting them in all my guns. I have had less consistent results using sub 5,000 psi loads so bump it up to 5,000 plus. In 20 bore I top off at 8,000 psi but could go 8,500psi. Pressures run higher in small bores. I shoot 2 1/2” in 2 1/2” chambers but accept others safely shoot 2 3/4” in their guns. It is just so easy to trim a bunch of hulls and load shorter ones, plus I enjoy the process.
