Richard,
Think about if for a moment. Say you load 3 drams of black powder in brass shell. No wad, no load. You ignite the shell, the powder burns off. Take the same shell and seal it. What happens when you ignite it? The pressure causes the shell to rupture. This is what happens when an obstruction causes a barrel to burst.
Many things can increase pressure. The load going down the barrel encounters friction. The more friction the higher the pressures. So wads play a factor as does the weight of the shot being pushed by the gases.
Pete
