As posted above, ammo lasts for decades with 100% reliability. Some really good info that others have posted that is very valid. Here is a quick summary (some of which other posters have already touched on).
Things that can shorten your ammo's lifespan:
- Water, humidity, and excessive heat are the main factors that inhibit/shorten ammo life.
- Some of the newer primers that do not use lead plausibly could pose a shorter life due to the new primers corroding much more quickly (time will truly tell).
- Sealed primers will probably outlast you in terms of service usefulness and lifespan.
- If ammo is not properly sealed, powder can clump over time but this pretty much only affects accuracy due to uniformity of ignition and burn.
Suggestions on storing your ammo:
- In a cool dry place..... haha. Ok what does that mean? Go ahead and store it in your closet (ideally not your garage or attic). Ammo and guns like about the same temperatures as you do. Also, I recommend keeping ammo at least 8" off of the floor in case you have an unexpected pipe break or leak in the house.. which will probably happen at least once in your lifetime if it has not already.
- Try Keep your ammo away from excessive oils/petroleum products (including WD-40). Not sure why someone would spray their ammo down with oils. But it does present the potential to foul your primers should they not be sealed in addition to staining your brass and they will not be as smooth (sometimes brass casings even get a little *sticky* when handled too much with oils).
- A good idea is to buy those cheap air-tight $5 plastic ammo containers. Placing your ammo in there will assist in the *dry* process though I caution you doing this if you are placing your ammo in adverse climates (excessive heat and humidity) as they have the potential of sweating. Throw in some cedar chunks or silicon packs for good measure.
- And if you want your 12 gauge shells to keep for a long time then don't stack a bunch of ammo on top of your shells... this is common sense but I almost caught myself doing this the other day when I was stacking cases in the closet.
Hope this was not too wordy but its what I have learned over the years and has worked well for me.