I see the question popping up pretty often:how long will ammo last?This is what happened to me and might serve to show how long ammo is good.
In the mid '70s,I bought a Browning BBR in 30-06.I developped a load consisting of a close to max charge of IMR 4350 under a 165gr Sierra BTSP bullet(chronoes at 2803FPS).The thing grouped 13/16'' at 100yds.
This afternoon,the weather was mild(low 30s),the sky overcast and the wind was blowing straight in our backs;a prefect day.My friend and I had the whole range to ourselves.Long story short,I posted a target at 200yds(4'' black dot)and let go at it.Exactly 1 1/2''.This from the same batch of ammo from 1976.Not bad for ammo 40 years old.I managed to save the last 10 rds to shoot them maybe 10 years down the road...if the Great Spirit lets me see that day.I'll report back if every round performs as it should!
Qc

Found an old coffee can full of 38sp. wadcutters my dad loaded sometime in the 60s, every round fired like it was loaded yesterday. No idea what powder or primers he used but knowing him I wasn't one bit afraid to shoot them.

I have a couple hundred rounds of 8X56R with a Nazi headstamp dated 1938. ALL fires just like new. I also have about 1000 rds of LC53 30/06 and it too fires like new ammo. If kept in a good GI ammo can in a constant temperature environment it will last generations. If hermetically sealed and kept in a cool dark place it will last a very long time.

The oldest ammo I have ever shot was 9MM largo Spanish milsurp from the 1920s or maybe early 1930w. It gave me maybe 2-3% misfires.

The oldest ammo I ever shot was some 30-06 military dated 1914...This was in about 1990. Every one fired. Every case neck cracked most partway down the shoulder. No I did not chronograph them. I have at this time most of a box of FA 45 Colt dated 1911..I have considered shooting some of them. I will chrono them if I do shoot any. I even have some inside primed 45-70 Military ammo. I'd like to shoot some of them...but can't bring myself to do it. I once reloaded some 12 ga aa epties on a PW 800 loader. I put them in original Win boxes and stored them on top of a 55 gal drum for more than 2 yrs outside rain shine heat cold snow etc in Md. When I shot them at skeet every one fired and I broke 50 straight. Amazing that they would still shoot.

How long modern, brass-case, non-corrosive, smokeless powder ammunition can be safely stored is, in my experience largely dependent upon the storage conditions. I was sort of a "living laboratory" for long term storage. Use this information for what it is worth to you.

I have been reloading since the late 1970's. In 1993, I was about to start loading 1,000 rounds of 5.7 mm Johnson (a/k/a 22 Spitifire) when I developed a neurological condition that almost killed me and I couldn't do anything in the way of reloading for 21 years. At the time, that 1,000 rounds of brass was in various stages of preparation and storage.

In 2014 when I went back to resume where I had left off, what I found surprised me.

• 500 rounds of unprocessed Norma .30 Carbine brass that had been in a Ziploc bag in 1993 were significantly and severely corroded. After extensively tumbling the cases to remove all corrosion that was confined to the surface of the case, I determined that 54 cases had been compromised and had to be thrown out. Another 51 cases were borderline and were retained for use in setting up my trimmer, neck reaming, experiments with "chemical milling" and other stages of the process that were unique to reloading a wildcat cartridge. So, just piled in a Ziploc bag, I experienced a 21% loss rate.
• 100 rounds had not been processed but was stored in two plastic slip top boxes. 15 of them had corroded so badly that they had to be discarded. A loss rate of 15%
• 200 rounds had been formed into 5.7 mm Johnson cases. They had been stored in compartmented plastic boxes. 15 of them had corroded so badly that they had to be discarded. A loss rate of 7.5%
• 200 rounds had been completed as loaded ammunition. They had been stored in compartmented plastic boxes which had then been placed inside a military type ammunition can. Two had minor corrosion that was easily and completely removed by using Brasso. Depending on whether you think I should have tried to rehabilitate those two cases, the loss rate was between 0% and 1%.

I attribute the difference in loss between the formed and unprocessed cases - at least in part - to some level of protection provided by the resizing lubricant. From this, I concluded that storing cases and loaded ammunition where they are separated from one another and not touching and in a sealed container is the best thing I could do to preserve my brass and ammunition. To hedge my bets, I also started including a small desiccant bag in each of the plastic boxes.

If properly stored..... basically dry ....It should last for a very very long time. I'm shooting ammo from the 50s on up

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I think a good test is to prime some cases and fill them with water. After a day try a few and see what per cent won't fire. Do the same thing the next day. Larry

The Largo ammo from the 20's, Probably had a misfire rate of 2 or 3 % when it was new! WWII Japanese ammo was made to last 75 years, The US ammo of the same time was meant to last 100! WWII German ammo was made to last for different periods. Navel 8mm was copper washed and sealed with saltwater proof bullet and primer sealer. I found the springs in the rifles don't last near as long as the ammo! At 75 years it is going great! Ammo late in the war going to the Russian front was expected to function in all weather environments except long term storage! Make it Now, cause we NEED it now!

Military NATO ammo, is most likely to outlast our children! Cheap look alike ammo most likely will be fine for 25 to 40 years.

Ivan

Keep ammo dry and in a temperature environment that would be comfortable to you and it is, for all reasonable intents and purposes, immortal.

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Pisgah

I have a number of factory WW2 issue GI 45 ammo still in the 20-pack. A bit shopworn, but every 10 or 15 years I put a few downrange just to see....

A few years back I was gifted a coffee can full of "machine gun bullets taken out the belt"....they all had military headstamp and an odd looking primer. I did try a dozen in a nice heavy Rooger revolver, and found some fired just fine & about 1/2 were inert.

I do have a few rounds left from 1979 when I first got into the reloading hobby. They still work just fine.

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Dum vivimus Vivamas

A little clarification please . Just what caliber of "Machine gun ammunition" that had been belted was it that you fired in your "Rooger" revolver. I am not aware of any caliber that would be belted for machine gun use that would be compatible with any revolver, regardless of how heavy the gun is. But then maybe what you said (typed) wasn't really what you meant!

For the OP, ammunition, factory or reloaded, will last virtually forever as long as it is not exposed to extremely unfavorable storage conditions. I still have several hundred .45 ACP "Powderpuffs" that I loaded in 1970-1972. I dig some out every once-in-awhile and have never yet had a mis-fire or hang-fire as of the last time I fired any. And I don't expect to have any problems with any of it in my lifetime!