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Some of the ingrediants used to forestall degradation of smokeless propellents have been found to last only 25 years under normal storage conditions. Storage in cool dry environments increase the useful life of propellents, but not in a consistent manner.
Cordite had an estimated shelf life of over thirty years, and some of this ammo has remained good for far longer. Cordite being extruded into sticks hard surfaced ans the surface moisture resistent may be the reason. The primers are the most common problem. The cordite requires a hot primer, the primer can degrade but still have enough flame to ignite as a momemtary hangfire.
Storage at temperatures over 125 degrees for any length of time causes the sticks to sweat nitroglycerin that pools in the case or soaks into the over the charge card disc if not well sealed.
Alternative cordite formulas were easier to produce in wartime, but were found to have a shelf life of around ten years. Most if not all ammo using these alternate propellents were either relegated to training or dumped in the sea after WW1.
Britain dumped hundreds of millions of degraded rounds in the North sea after each World War. Millions more rounds were sold to manufacturers of wood glues and furniture finishes the propellents broken down for their chemical components, the brass and bullets then sold as scrap, sometimes bought up by small firms who remanufactured the ammo using new primers and powders.
I don't trust surplus ammo, even that no more than 20 years old. I've found terribly degraded 7.62 NATO ammo with the headstamp of a often recommended manufacturer.
Only use I'd have for surplus ammo is to break it down for components.
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Temperature accelerates the migration of nitrogylcerine (NG), but it is the action of the polar molecule H2O that wicks it to the surface of the grain.
When the nitroglycerine is wicked to the surface, it changes the burn rate. In double based powders the nitrocellulose and nitroglycerine are mixed as evenly as possible, but when the surface becomes NG rich, the pressure curve spikes.
Even though some of the "experts" served in the Army, maybe they did not have a need to know, or probably just did not care to find out, but the Army, Navy, Air Force and Marines all have stockpile surveillance programs, which covers all propellants, to weed out and get rid of the older stuff. It causes a major scandal when a facility goes up in flames and explosions,
http://www.liveleak.com/view?i=13c_1205681217 and it creates paperwork when a soldier is injured with old ordnance.
Rocket motor propellant is usually a double based powder with more herbs and spices, and if you talk to the community, old motors have a history of going kaboom!. The shelf life is 20 years, then the things have to be re certified by X rays and visual inspection to determine if the propellant has collapsed, or if there is NG pooling on the surface. Recertification is expensive and is not 100% successful, because the motors are past their design life and many have to be scrapped anyway, and the frequency of inspection increases because propellant naturally breaks down. Any one remember which law of thermodynamics applies?
Still, I am waiting for the "experts" to tell me how single based propellants are not effected by age and storage conditions.