Here is a bit more precise explanation of the chamber pressures involved in this accident. This is from the Double Gun BBS and is in response to question from a non-reloading shooter that does not know one powder from another.
" Paul intended to load black powder in his shells. A charge of around 74 grains (about 1/6 ounce) would have produced near 7000 psi in the breech of the barrel. There is a nitro base powder named PB that is often used in shotshells, especially to make lighter loads with low pressure. PB looks a lot like black powder on casual inspection. However, the energy content of the PB is such that a charge of around 20 grains (1/22 ounce) will produce a load similar to the black powder. Having mistaken the identity of the PB for black powder, the shells were loaded with a weighed charge of 74 grains, a 3.7 times overload. This overload of powder produced a breech pressure that was probably in excess of 30,000 psi, about 4 times over what was intended. That the gun burst is no surprise. That it endured three with bursting is a surprise."
Lars