Smokeless Powder Deterioration
I've read what Dr. Tenney L. Davis's 1942 textbook "The Chemistry of Powder and Explosives" has to say about this, and I have a US military article that I downloaded before they took their technical stuff offline or at least blocked it from search engines. Nitrocellulose is inherently unstable. You can't make it stable, but you can reduce conditions that make it more unstable. There is a small amount of water left in all powders, and this gradually reacts with nitrocellulose by hydrolysis, a molecule of water reacting with a nitrate group on the nitrocellulose to produce a molecule of free nitric acid and restoring the original hydroxyl group of cellulose at that site. Nitric acid catalyzes more rapid hydrolysis of other nitrate groups, which accelerates exponentially. You might think that removing all the water would be a fix for this, but it's not practical and a very thin film of adsorbed water on the surface of the powder grains actually helps extend the powder life. Nitric acid decomposes to produce nitric oxide, which is clear, and nitrogen dioxide which is the nasty brown gas. Same stuff that makes smog brownish. The nitrogen oxides also catalyze more rapid breakdown of nitrocellulose. Lots of things have been tried or used to block these reactions, but two have been widely used. Diphenylamine reacts with the nitrogen compounds to form nitrodiphenylamine, and can react once more to form dinitrodiphenylamine. When all of the diphenylamine is used up this way it no longer helps to preserve the powder. Usually about 1% diphenylamine is used. More than about 2% diphenylamine deteriorates powder faster itself. The other acid neutralizer is calcium carbonate in the form of chalk, put in the aqueous phase during Ball powder manufacture to clear out residual acids from either newly made nitrocellulose or old deteriorated powder being recycled. Some of it gets into the powder, where it can continue to absorb nitric or nitrous acids released from deteriorating powder. That residual chalk has also been reported to cause gas system fouling and bore deposits in some cases, so since the mid '60s the amount of it in the powder has been reduced. The other big thing that deteriorates powder is heat. Like most chemical reactions, these reactions occur more rapidly as the temperature rises, often exponentially. Powder lasts much longer with cool storage. The military has used a system where a sample from each batch of powder used is kept, referenced to the ammunition lots loaded from it. These samples are kept at an elevated temperature around 150 degrees F if memory serves me, and they have a formula for this more rapid aging to convert to expected ammo life in normal service. They watch for red fumes, and when that starts the powder is done. When they predict a lot of ammo has expired according to the test they retire it, it's pulled down and that's where much of our surplus powder comes from so it's already on a short expected life. Furthermore, the decomposition reactions are exothermic, releasing heat. If powder is stored in large bulk amounts the heat can't escape, the rising temperature makes the powder decompose faster releasing more heat, and eventually the powder autoignites. This has caused major disasters with battleships, ammo dumps etc. blowing up. That's a possible consideration if you have barrels or cases of powders in close storage. One more thing about deteriorating smokeless powder: It loses chemical energy as the nitrate groups are lost, but often old powder makes erratic, unexpectedly high pressures. The reason, according to the Army ballisticians, is that the long molecular chains of nitrocellulose break into shorter molecules. This makes powder more brittle. Powder grains shatter during firing, exposing much more surface area to burning, making it burn faster and raising pressures. That is a major reason they retire military ammo when it reaches the end of its "best used by" life. Very old ammo may exhibit this. I've seen signs of higher than normal pressures including perforated primers with early 1940s Turkish ammo, which has likely seen high temperatures in storage. So check your powders now and then for the red-brown fumes. If you see any it's time for that powder to be safely disposed, and you'd better make sure any ammo loaded with it gets used quickly or broken down.