So I'm seeing that old ammo is not as stable as old powder. Perhaps has something to do with the powder in ammo being in brass? Or perhaps the priming compound affects it over time? Interesting
Maybe I have confused people.
Gunpowder is an unstable high energy molecule that is breaking down from the moment it is made. Heat accelerates the breakdown. As gunpowder deteriorates it gives off NOx. This NOx attacks nitrocellulose accelerating the deterioration process. Gunpowder has stabilizers to soak up the NOx but at some point, they are consumed. When gunpowder breaks down it produces heat. When the stabilizers are depleted and the reduction-oxidation process starts ramping up, gunpowder can produce enough heat to self ignite. All it takes is a tiny spot to be hot enough and the whole pile burns.
Big piles of old gunpowder have the potential to catch fire. Their surface also becomes sensitized which means that friction alone can cause it to ignite. I believe this explains the turret explosion in the Iowa. They were using old WW2 era powder that had been stored in hot conditions for years and was fuming as it was being rammed.
As gunpowder breaks down, producing NOx, the powder grain deteriorates and it deteriorates unevenly. At the same time some of the NOx reacts with water vapor (always in the air) to become nitric acid gas. Nitric acid attacks brass, it weakens brass, it also causes corrosion. I don’t know if the bitter smell is from NOx or nitric acid gas, probably from NOx as it is a series of compounds.
So with old powder you have this uneven powder grain that will not burn in a consistent manner. I was told it causes “burn rate instability”, the gist of which is that you don’t get a smooth pressure curve. If the burn is uneven, pressure waves can get out of sync with each other and that will cause pressure spikes. Given that the cases have been weaken by nitric acid gas, high pressure and weak cases are a bad combination.
If the powder physically breaks down, to a dust, the surface area is vastly increased, and the initial burn rate will be high.
If the powder is double based, nitroglycerine attacks the nitrocellulose, increasing the rate of deterioration. This is why double based powders have a shorter shelf life than single based. Nitroglycerine will also migrate to the surface of the powder grain, because water molecules, condensing and evaporating in the air, wick the stuff to the surface. A nitroglycerine rich surface will change the burn rate, actually spiking the pressures.
I don’t know that much about primer deterioration. Primers compounds are tested to be compatible with gunpowder. The old chlorate primers used to dud out over time. From what I have read heat will dud out styphnate primers, but I don’t have a good idea for the shelf life of lead styphnate primers.