Questor,
ABSOLUTELY no danger from keeping old cartridges around. Unless you are talking about really LARGE quantities of specialized military ammunition (explosive warheads, incendiary, illumination rounds, etc). You DON'T have any of THOSE, do you??
Old, decayed dynamite sticks are hazardous because the nitroglycerin leaches out of the inert filler (diatomaceous earth) and forms crystalized nitroglycerin. Unstable, as any accumulation of nitroglycerin is. It is highly sensitive and explosive, of course.
Small arms ammunition, on the other hand, when it gets old, the primers become less sensitive, and eventually become inert "duds". The priming compound "kills" itself and is no longer sensitive to impact detonation. Nitrocelluose and nitroglycerin (single or double base "smokeless" powder), when it deteriorates, breaks down into rust-coloured dust, but it is not unstable. It is still flammable of course, but is contained in the cartridge case.
Old ammunition that has poor storage or been subjected to tropic heat and humidity CAN break down into corrosive components (residual acids) that will corrode thru brass cases. Still not dangerous, but messy. This deterioration will take many years, usually several decades. The US Military considers 25-year-old small arms ammo still suitable for training use. Surplus WWII ammo is still routinely shot up for plinking or recreational shooting.
Military storage of large caliber cannon ammunition or large amounts is different. Deterioration of large amounts of powder can generate spontaneous combustion hazards. For those reasons, large stores of smokeless powders are constantly monitored for internal heat, and is unpackaged, inspected, and repackaged regularly.
HTH
John
