Lets start off with the rule of thumb for the lifetime of propellants: 20 years for double based and 45 years for single based. Powder breaks down naturally, it is a high energy compound that is breaking down to a low energy compound from the day it leaves the factory.
Nitrocellulose-base propellants are essentially unstable materials
that decompose on aging with the evolution of oxides of nitrogen. The
decomposition is autocatalytic and can lead to failure of the ammunition or disastrous explosions.
ROLE OF DIPHENYLAMINE AS A STABILIZER IN PROPELLANTS;
ANALYTICAL CHEMISTRY OF DIPHENYLAMINE IN PROPELLANTS
Heat greatly reduces the lifetime of ammunition, see table one in this manual for just how much:
Surveillance and in-service proof - the United Nations
Heat will age powder and create pressure issues:
INVESTIGATION OF THE BALLISTIC AND CHEMICAL STABILITY OF 7.62MM AMMUNITION LOADED WITH BALL AND IMR PROPELLANT
Frankfort Arsenal 1962
3. Effects of Accelerated Storage Propellant and Primer Performance
To determine the effect of accelerated isothermal storage upon propellant and primer performance, sixty cartridges from each of lots E (WC 846) and G (R 1475) were removed from 150F storage after 26 and 42 weeks, respectively. The bullets were then removed from half the cartridges of each lot and from an equal number of each lot previously stored at 70F. The propellants were then interchanged, the bullets re-inserted, and the cases recrimped. Thus, four variations of stored components were obtained with each lot.
Chamber pressures yielded by ammunition incorporating these four variations were as follows. These values represent averages of 20 firings.
Old ammunition will blow up guns through burn rate instability. This is analogous to using old gasoline in your lawn mower. Do you remember how that engine ran rough, back fired through the carburetor, and if you had continued, maybe something bad would have happened? What you want from gunpowder is a smooth pressure curve. Since old gunpowder burns irregularly, it can create pressure spikes. I found gun blowup accounts in my 60’s American Rifleman magazines, one guy shooting old WW2 ammunition blew his rifle all to pieces. On disassembling his rounds the writer found clumped powder. You don’t want hangfires, hangfires are a very discontinuous burn, the pressure waves are reflecting and rebounding, irregular pressure increase beyond the combustion structures of the gun, there is risk in hangfires.
Only Superman has X ray vision, so you won’t see the corrosion going inside of the case less you pull the bullets, but if you ever pull the bullets and see green corrosion, like what is on these pulled US surplus bullets, that powder is outgassing nitric acid gas and is beyond its shelf life. It probably will go bang, but the case will be weakened and you could be unlucky and have a case head rupture, you could also have pressure spikes due to burn rate instability.
Incidentally, double based powders, the nitroglycerine wicks its way to the surface, so the surface in nitroglycerine rich and that will spike up the pressure curve.