raythemanroe said:
Okay explain why pressure increases drastically when under loaded if deflagration is still happening?
The early stage of firing a cartridge is very complex... and it is a balance of many things in order to make it work.
When a bullet is fired, it leaves the case neck and jams into the throat/leade of the rifling and for a minuscule moment, it
slows, while the gas pressure is building up high enough to force it into the rifling and on out the barrel.
This is a critical time, because if the bullet stops, it is extremely hard to get it going again, because it is stuck in the throat, and something called "sticksion" (stik-shun) takes over and then it takes a humongous force to get it going again. Sticksion was discovered by John Bell Blish, when he was studying large artillery. It is the same phenomenon that causes the walls of a cartridge to stick to the walls of a chamber when it is fired (leading to head separations).
http://en.wikipedia.org/wiki/Blish_lock
When a bullet is in the case and the powder is first lit, normally this stage is the time that the powder also gives off the most gas, because the particles have the largest burning surface area... but since the bullet is barely moving, this is the time when you want a limited/controlled amount of gas given off, so you can start the bullet into the rifling... then, once the bullet is moving faster, and there is a larger volume of space in the bore to fill... then you want more gas given off.
Normally, smokeless powder works the opposite way - it gives off the most gas in the beginning, and then gives off less and less gas until powder is gone.
So, powder has a coating called a "deterrent", which slows the early burning stage to let the bullet get moving. Then, when the flame consumes the deterrent, the burning gets faster so it can generate gas at a faster rate to keep the bullet accelerating out the barrel.
So, the normal sequence is... the primer "lights" the powder, and the bullet gets started... it moves into the throat/leade and the powder gives it slow, steady push past this early resistance, and the bullet starts moving in the rifling... the deterrent is consumed and the powder starts burning much faster, but there is more space for the gas in the bore, and the bullet is now moving fast, and is on it's way.
Now, this scenario is possible when a large case, long bullet, and light load of slow burning powder is fired....
With a large case, and a very long bullet with a long bearing surface, and a small amount of slow burning powder... the primer lights the powder, but the pressure in the case is low - the powder pushes the bullet into the throat/leade, but the small powder charge has not created enough gas to keep the bullet moving and it stops.
Now the bullet is stuck in the throat/leade, and the small amount of powder burns through the deterrent coating and now starts to burn faster... and the bullet is still stuck. The pressure keeps rising and the powder keeps burning faster and faster (because that is what "progressive burning" powders do), and even if the bullet breaks free, it is now moving slowly up the first few inches of the barrel, while the powder is burning fast so it can generate the volume of gas that it should, IF the bullet was accelerating half way out of the barrel, the way it should have been.
So stuff lets go.
Some people have complained of bolts being hard to open with light loads of slow powder, of have had other symptoms of very high pressure,, while using light loads of slow powder, and heavy bullets... but no one has been able to blow up a gun in a lab, that can be traced to the Blish effect.
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That is how it works.
