Good gunpowder should have a 20 shelf life, but it is not as simple as that, Armies would not be spending money having Ammunition Specialists go through their munitions and culling out the rounds that are going bad.
this is from a 1969 Insensitive Munitions presentation
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more recent
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Gunpowder has a "unknown" lifetime. That is, it goes bad, and the only way to find out if it is bad, is through inspection.
You know, none of the people in my gun club, noticed these warnings. Or understood their significance. These lots went bad, and probably burnt a few houses down. And it was not simply because the powder was "bad", it was because the powders went bad.
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Break the seals and inspect your stocks when you think of it. You don't want this:
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you don't want this
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this happens
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and this happens
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this was very bad and it was lucky none of these tins autocombusted
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My advice, break the seal, and sniff. This is not 100% perfect, but lacking a gas chromatography machine, it is what you have. If the powder smells bitter in the slightest, get rid of it. It is not perfect because I have had powder that cracked case necks within months of being loaded, and the stuff had no foul smell. Still poured it out. But if you ever sniff a foul, bitter odor, pour the stuff out. If you ever run across something like this:
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Get it out of the house immediately. If you sniff, that red stuff is nitrogen dioxide, and it will rip the tissue out of your nose. And, what you don't see, is nitric acid gas that is created when NO2 bumps into a water molecules. Molecules are very tiny, by the way. Incredibly tiny. The combination of the two may cause damage to your lungs. A sniff will knock your socks offs.
When powder has gotten to this state, it is ready to autocombust. It is very dangerous. Surveillance of your gunpowder stocks is important, if you are concerned about waking up dead.
You will find a lot of misinformed shooters who think if they never break the seal, the gunpowder will somehow last forever. They are wrong. Some of these guys are proud to share pictures of their WW2 era powders, sold by Hodgdon.
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They have no idea the state of that powder, and you know, stuff that old is dangerous to store in the house.
By the way, these are various tests the Army was using back in the 1960's, to determine the remaining shelf life of gunpowder.
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The shooting community does not want to know this. The eraser of this knowledge, that gunpowder, and thus, ammunition does not last forever, has been willingly and deliberately erased from the shooting community. I think it is due to hoarding behavior, and the delusion of immortality. Everyone thinks they are going to live forever, therefore it only makes sense, that your hoard of gunpowder and ammunition lasts forever. Since you are going to be around forever. Totally delusion and irrational, but that is my guess. I have had very angry, insulting reactions when I tell shooters that their powder has gone bad. I think the angry push back is because, if their powder is not immortal, then in some way, I have taken the re loader's immorality away too. In a way, I have murdered them. Not a way to make friends! Whatever is behind this, the shooting community wants to be stupid, and industry is happy to keep us stupid. It is a win-win for them. They don't make money telling
us what not to buy. And, a stupid shooting community is not going to be picky about the age of ammunition or gunpowder. Ignorance is strength, from the industries viewpoint.