Note that when we speak of minimums, this is not necessarily meaning anything thinner will "Burst", but allows for a "Safety Factor". W W Greener reported on page 579 of 9th edition turning a 12ga bbl down till it measured .050" at chamber/cone juncture. It Bulged "Slightly" after "Repeated" firing with 3¼ drams of "Best Alliance #4 powder with 1¼ oz shot.
It is also I think worthy of note, that formulas for determing hoop strength vs pressure are nearly always calculated for "Static Pressure". When pressure is applied to steel it has to first "Stretch" beyond the elastic (Bulging) limit & then Ultimate (Bursting) limit. This can be somewhat understood by realizing how pressures recorded simultaneously by the Lead Crusher & Piezo Electric Tranducer methods do not record the same pressures. The time the pressure is applied is of such short duration even the lead is not compresed to the extent the pressure read by the tranducer would indicate.
Upon firing a shell thusly the bbl does not "Swell" to the extent it would if the same pressure as the max pressure generated were applied as a static load.
