The question: 

Why Does a Short-Started Ball Cause a Barrel Bulge? 

by Dr. Stephen Wardlaw
If a barrel is obstructed somewhere past the powder charge, the barrel will bulge or blow out just below the point of the obstruction, To understand why this happens, one must remember that the barrel is filled with gas. A very hot gas from the burning of the powder, but a gas nonetheless, and in any gas, a change in pressure (think of it as a change in direction of the gas) can not exceed the speed of sound in that gas, and this speed depends on the temperature and average molecular weight of the gas. 
As shown in the diagram above, in the normal course of shooting, pressure builds up in the barrel and accellerates both the powder gasses and the projectile. The gasses behind the projectile move as an expanding column, where the gas molecules just behind the projectile are moving along in step with it, while gasses in the breech pretty much stay in place. All elements are moving smoothly, and with sufficient powder charge, the projectile and the expanding gasses behind it can easily exceed the local speed of sound. Air trapped in front of a projectile which exceeds the speed of sound can't move out of the way fast enough and is compressed and forms a shock wave. When the projectile exits the barrel, the shock wave causes the characteristic 'crack' of a rifle when the muzzle velocity exceeds about 1100 fps. 
Now let's take a look at what happens in an obstructed barrel, for example, when the projectile is seated only half way down. When the powder charge burns, it creates an expanding column of gas just like it normally would, but it expands faster, since there is no projectile to push. However, the leading edge of the pressure wave can't travel faster than sound without building up a shock wave in front of it, so the first thing that comes into contact with the obstruction is this shock wave. Unless the obstruction is extremely light, it will slow or stop the shock wave for a few milliseconds, which is all it takes to do the damage. Since the gasses travelling just behind the shock wave are moving at the local speed of sound, they 
are unable to quickly reverse direction, and most of the column of hot gas plows into the area behind the obstruction - just like a multi-car accident on a freeway, where the cars simply can't stop fast enough. This rapid build-up of gas equals a rapid build-up of pressure; it is almost the equivalent of setting off the powder charge in a closed container, because the gasses are confined in front by the obstruction which can't move out of the way fast enough and in the rear by the gasses coming up the barrel which can't stop fast enough. When you think of it this way, it is no wonder that even a small charge and a light obstruction can ruin a barrel. 

The moral is - Never give those powder gasses a running head start; always keep the projectile seated on the charge.