While closing out the estate of my parents this year and the estate of a late aunt and uncle a couple years ago, I came across a pretty wide range of ammunition, most American, the rest European, in a variety of calibers. Uncle Huke had brought most of it home after WWII, but some was old hunting ammo, made here, in the 1930's. My dad brought home smaller quantities from his service in the Navy in the Pacific.
I went to a bit of trouble to clean the cartridges up. They had been stored in open crates of other odds and ends, in unheated garages, winter lows reaching zero frequently at night and daytime highs topping 100 degrees a dozen times each summer. Kept pretty dry and our regular humidity is quite low.
I rounded up rifles and handguns to shoot this hodgepodge of material. I had guns for everything except the 20MM stuff, most mine, some borrowed, some who brought a gun so they could shoot it with my old ammo.
This stuff all pre-dated the end of WWII.
It all fired reliably. Some 8X57 rifle ammo took two strikes from the firing pin to discharge but that was the only problem. Lots of smoke compared to current loadings. They all had plenty of energy to cycle the self-operating weapons.
The largest quantity if ammunition of a single type was USGI .45 ACP ball. They were steel case, from the Evansville Chrysler ammo plant made in 1943. I had several dozens boxes of 50 of those. The cardboard boxes had gotten wet and pretty much disintegrated, leaving the cartrdges corroded to varying degrees.
I tumbled these .45's in hot soapy water to remove the shreded cardboard and it's accompanying mouse turds. I tumbled them in walnut hulls after that. They remained somewhat disclored and a few had some minor pitting.
I shot all the .45 ACP ball ammo in one session, using a mint Remington Rand 1911-A1 a deputy sold me. The ammo all shot and cycled the handgun just fine. I shot each 7 round gunfull into a paper plate at 25 yards and rarely lost one strike off the edge of any plate. Ignition seemed consistent.
When returning a Mauser K-98 to the owner who loaned if for the test, the owner was pleased to hear that all of the 8X57 ammo fired well and with powder. He referred me to a mutual acquaintance who had some even older cartridges for us to test.
A short trip brought us a nice lunch of German bratts and some other Teutonic treats before he produced a very nice Winchester 1917 and a near-mint 1903 Springfield. He then dragged out some 500 .30-06 cartridges, made by various armories, some dating to 1914, but most dated in the early 1920's. Many had been stored in cartridge belts and pockets and had corroded a bit with green crud sprouting all over. I tumbled these in coarse sand until most of the green was gone and then in walnut shells. I discarded about 15 or 18 rounds whose brass looked like too much metal had been eaten away to shoot it safely.
We shot all of the 500 rounds of .30-06 in two afternoons. Every round fired on the first try. Every bullet struck the humanoid portion of the B-27 target we placed at 50, and then 100 yards. There were a few dozen with split case bodies and necks, but no hangfires at all.
I like to shoot the FN-FAL rifle and its variants; at one point, I had 14 of them, plus a CETME, an HK-91 clone and an M-14. So, of course, I shot up a lot of milsurp foreign-made .308 Win/7.62X51ammo. During the late eighties to the early 2000's, importers brought shiplods of it, and other surplus cartridge, into the U.S. I bought Spanish Santa Barbara, British Radway Green, Austrian DAG, Germen MEN, Portuguese FM, Australian, South African in crates of 800 to 1,260 rounds per case. A lot of this was headstamped 30, 35 years earlier than I got it. It came from all over the world and the cartridge crates, plus the cardboard boxes and plastic sleeves they were in, took real beating. Many crates showed extreme weathering. Some had termites get inside the wood crates and then invade the plastic "battle packs" that held 200 rounds in 10 paperboard boxes. This ammo came from all over the world. The packaging showed it had taken a beating and got rained on, snowed on and baked in the sun for 30 or 40 years, and it is just fine, not degraded. It has all fired for me.
The oldest cartridge I shot was a .45 Colt cartridge with a copper case and internal priming (the case shows no hint it has a primer, examined from the outside) filled with black powder. This had to be a late 1880-1890 product. I thought about shooting it in my Schofield First Model, but since I don't know how old black powder could act, I shot it in a borrowed Ruger Redhawk instead!
All this occurred over the last 10 years. Most of the rounds were at least 65 years old. The .30-06 was 91 and 82 years old, while the .45 Colt was probably more than 100 years old. I wish I had tons more of it to shoot.
I also had the experience of carrying handgun and rifle ammo in the trunk of my police vehicle. I kept track of what sidearm each deputy who worked for me carried, and assembled loaded magazines for each. I got enough of the department's issue ammo and filled at least 2 magazines with 9mm Luger, .357 Sig, .40 S&W, 10MM Auto, .45 ACP and a couple of M-16 30 rounders with 5.56. I then used bubble wrap and nylon strapping tape to make a pouch for each magazine. If a deputy was pinned down and running out of ammunition, I hopefully would be able to throw the bubble-rapped magazines to them.
I carried the magazines and my O.C. pepper fogger in a zippered gym bag in the trunk of my Crown Vics or in the cab of my F-150. It was sarcastically labled "PARTY FAVORS."
Those magazines were carried in the trunk of my various Crown Vics and the cab of my F-150 from July, 2003 until July, 2009, when I shot all of them up and drew a supply of fresh cartrides. I chronographed a few of each batch, and they shot as fast as they did before riding around in a cop car for six years, none of the time garaged. Those 90 2.23 rounds shot just fine, as well. Six years of hot, cold and the bashing and vibration up and down Salt Lake County's horrible streets didn't damage them at all.
Black powder MAY degrade with age and become less powerful, but modern 19th and 20th century ammo seems to last as long as we could hope for.
My department still has us use up our old duty rounds during a training session annually and issues us a new ration of fresh ammunition "just in case.'
I really don't think that modern loaded cartridges have a shelf life of much under 100 years, generally, if the storage is at all reasonable.