I'm not sure why I posted here. I knew better, but did it anyway and now I kinda feel like I need to justify/explain some of what I said. I originally responded to Justins question with the assumption that he was already loading NFB rounds with Dacron filler and knew the potential hazards involved. Therefore my statements were made with that in mind. I also used the word 'must" in one of my posts which we all know is not true. I used it in the same way as I would when telling someone that they must look both ways before crossing the street. They certainly don't have to, but it would be foolish not to.
With that out of the way I work in the explosives field and have for my entire adult life. I manufacture, test, and dispose of explosives for a living. I've been lucky enough to have a very diverse career that has allowed me to work for several different companies and agencies giving me a much more rounded knowledge base than many others. I don't know everything but I when I speak to these things I do so with a lot of experience and education to back it up. I also load for a lot of guns, including doubles, both with and without fillers.
I'm going to try and write a few things here to hopefully shed a little light on things for some people. I'm going to write this as a forum post though, not as if I'm writing a peer reviewed paper for a scientific journal. Things will be (sometimes) overly simplified to make things easier to understand and to save me time. I'm writing this to try and help others not to help myself. If people have serious questions I'll try to answer them if I have time. If people want to argue I'm gone.
Many people are referencing studies that only looked at one variable and then intermix these studies trying to come to the conclusion they want to. This is a large part of the problem. Vieille and Dell say they can ring a chamber with just powder yet Bell couldn't. Why not? The answer is that the powders used were very different. Explosives are generally rated by "power" and "brisance." For this discussion power will be the total amount of gas created by the explosive. So if 10 grains of powder X creates 1 cubic foot of gas it's twice as powerful as powder Y that creates .5 cubic feet of gas with the same 10 grains. Brisance is the speed at which those gasses are created. They are both important but do different things. 10 pounds of black powder will be more powerful than 1 pound of RDX. If I place these two explosives on a 1 inch steel plate the larger charge of BP will just bend the plate while the smaller RDX charge will blow a hole through it. The brisance is what cuts through the steel. With smokeless powder the basic makeup is more or less the same and therefore the power is very close to the same for the powders reloaders use. Additives are added to slow the powder down and lessen it's brisance and this (along with grain shape) is what changes where they fall on the burn rate chart. A case full of Bullseye doesn't necessarily have more power than the same case full of 4831 but the brisance is totally different. This causes the pressure to spike more quickly with the bullseye to where things fail even thought the total energy (power) is the same.
So...if you use a faster powder you are more likely to ring a chamber as the brisance is higher and it creates a stronger shock wave (which doesn't necessarily correlate with pressure) which can ring a chamber as Vieille found out. When an explosive detonates it creates a large cloud of gas which radiates outwards. The leading edge of this cloud is where the shock wave lives and it's what shatters things it comes in contact with. It can bounce off things and it can compress things (including air). Since the shockwave radiates out from the explosive, if a case is pointed up, the wave will radiate towards the bullet since all the powder is at the base. This will cause a convex wave to move towards the bullet (which is an obstruction) until the wave hits it. The whole time it will be compressing the air in the case (since gas is compressible). When the top of the wave hits the bullet in a straight walled case you will have all the compressed air (and a portion of the shock wave) forced into the bullet case junction by the shock wave. Remember it's convex shaped so it will be forcing everything into this area as it moves forward. This is why the ring forms where it does. If the powder is laying on it's side when the cartridge is fired the wave will form differently and start at the base of the charge and move forward and sideways from it and will impact the bullet in a very different way. This would explain why chamber ringing wasn't seen when the charge was fired in this way.
So what can we do about it? Use slower powder. When you use slower powder we generally use more which has less brisance and fills more of the case so we have less air space. Two birds with one stone. We all know that at a certain point it's too slow to reliably burn though so we either need to go back to a faster powder, shorten the case, or add something to fill up the space in the case and/or hold it against the primer so the powder can stay in contact with it long enough for the flame front to encompass it.
Placing a small amount of something over a charge of fast powder probably makes things worse as it holds the charge against the primer longer allowing the flame front to travel quickly. This causes the shock wave to form perfectly at the furthest end of the case and ensures all the powder burns. I myself would not do this although I know some people do and haven't had issues. As you move to a slower powder the risk of doing it this way is reduced and you can get a cleaner burn with more consistency. I still personally choose not to do it this way.
There's also another thing we need to look at when we introduce fillers. Just as the bullet is an obstruction to the shock wave it's also an obstruction to the filler which will be propelled into the stationary bullet at extreme speed. This can also ring a chamber in the same way as an obstruction in a barrel creates a bulge when you shoot a bullet into it. The problem is how do you determine which one is doing it and how much one contributes to the other? This is not something you can figure out with 100 tests. It would be much more involved and very expensive. Some fillers are frangible and break up (likely by the action of the primer) and may not cause as much harm as another that is solid. I can speculate but NO ONE can tell you for sure without a lot of testing. One thing I can tell you is FILLING the case with filler (when you choose to use one) has advantages. First it will take up space. You're substituting a solid for the air in the case. The more you put in there the less air. The second benefit is that if you put in enough filler that it is touching the bullet base as the charge pushes it forward pressure is progressively raised until the bullet starts moving. We all know throwing a baseball hurts less than slapping one. This is how I choose to use fillers knowing what I know.
This is just the basics. There is so much more that influences this discussion. Different grain sizes and shapes will be easier to ignite than others. The primer will disperse some powders through the case in different ways than others prior to full burn. This will effect things. Bottleneck cases will redirect the shock wave differently than a straight walled case and will also compress the filler more in this area. This will change things.
In the end there's only a few things anyone can tell you for sure:
You can ring a chamber with powder alone.
You can ring a chamber using fillers.
You can load with just powder and not ring a chamber.
You can load with fillers and not ring a chamber.
I hope this helps some people understand how things work and explains why you don't have the powder companies giving you the magic formula to alleviate this in all circumstances.
