Instability has to do with the powder breaking down into its component chemicals (e.g. cheap aspirin breaks down leaving acetic acid, hence the vinegar smell in a big bottle of cheap aspirin). It MAY be harmless, but IF it breaks down into an acid or a less stable oxidizer (e.g. chlorate) or heaven forbid both, it may spontaneously combust or heat up. At any rate, once it has decomposed it will not fire off properly. I do not know the decomposition products of cordite, having never dealt with it, but both my husband and I used to be fireworkers, and decomposition is an ongoing and dangerous problem in fireworks.
As for cordite, I find in Chapter 15 of Sharpe's "Complete Guide to Handloading" that the British patent stuff is 30% nitroglycerine, 65% guncotton, and 5% petroleum jelly, with acetone as a solvent. I have no idea how unstable it is, (although guncotton is notorious unstable) but it is certainly sensitive, that is, it is subject to ignition through static electricity, impact, or being struck with a metallic object. (Modern smokeless powder is mostly made of nitrocellulose). If you have any more of that stuff around, THROW IT AWAY!
Hey, thanks for the info. I never had that explained to me before. I never knew the products of decomposition could themselves be dangerous.
As for the decomposition problems of fireworks, I can certainly verify that. I got some bottle rockets that are 10+ years old. They are a little hair-raising to shoot off since you don't know what they're gonna do...sometimes they just blow up like an inch and a halfer and sometimes they lift off only a foot and then drop to the ground. As long as you are careful, and keep your eyes protected and wear good clothing(cotton only), you are ok. Being an avid motorcyclist and having some experience as a welder, I am in the habit of wearing protective clothing at all times regardless of the heat and humidity. Having fair skin that sunburns easily is an added incentive for me as well. Bottle rockets aren't too awfully powerful. Even if one went off in your fingers, it is still harmless.
Oh, no offense, but I think you are over reacting a bit about the old british chordite. Real black powder is also very sensitive and lots of people still use it, including myself.