I've fired a lot of old U.S. military ammunition over the years. Originally just for the fun of shooting guns but later for curiosity. I'm very interested in U. S. military arms and have tried to gather at least one example of each major issue piece.
When I was young, World War II .30-06, .30 Carbine, and .45 ACP was commonly available in the circles I ran in. To fire World War II dated .30 Carbine ammunition in particular and savor the odor triggers strong memories of being a kid and shooting my dad's and Uncle's Carbines out at the lake or around our house.
The earliest military ammunition I've ever fired was some Frankfort Arsenal '04 dated .30-40 ammunition. I've still got a complete unopened cloth bandoleer of it, still stitched up, I'm keeping as a collectible. I fired a dab from a second opened bandoleer over the chronograph just to see what it would register (1958 fps).
I've also done the same thing with U. S. Military Contract .38 Long Colt ammunition from the Frankfort Arsenal dated 4-'11. It clocked 671 fps (4-shot average).
Have also fired some U. S. Military Contract .45 Colt ammunition dated 8-'11. It gave a velocity of 723 fps (a single round).
25-30 years ago I shot some World War I dated .45 ACP ammunition from a couple of different contractors and seems like I recall off some Remington 1918 dated .30-06. I shot up a batch of FA '33 .30-06 ammunition back in the 1980s. Still have the cases around here. Used them with handloads a few times.
The earliest ammunition I have ever fired was some late 19th century Winchester produced .38 WCF (.38-40). This had an early style head stamp and curiously, featured a small rifle primer. It was loaded with black powder. I acquired a little less than 300 rounds from a pawn shop. It was loose with no boxes and very dirty. This was about 1980 and that was when the ammo was fired. I shot off perhaps 100 rounds, of which only half would fire. I gave some more away and still have the remainder around here somewhere, including all the rounds that didn't go off. Every once in awhile I encounter them when I'm digging through stuff looking for something.
The problem with using old ammunition for shooting purposes now is that some of it could be collectible. It can be a bit fragile, or at least possess brittle cases. The .30-40 ammunition came out of the bandoleer, bright and shiny yet some of the case necks were cracked, one supposes from the tension of the bullet having been seated for so long. Nearly every case fired showed a crack in its neck afterward.
All priming compounds prior to World War II were corrosive in nature; some highly so. Only World War II era .30 Carbine ammunition was uniformly produced with non-corrosive primers. Some lots of military .30-06 were still corrosively primed into the early 1950s. I've heard these were lots of specially prepared match ammunition.
Bullets were jacketed in cupro-nickel until the mid 1920s. The .30-40 Krag ammunition I shot features the most beautiful silver-hued 220 grain round nosed bullets you ever saw as does any World War I .30-06 ammunition. The metal fouling that cupro-nickel leaves in a bore is said to be of a most insidious variety. It supposedly clumps up and then the clumps gather more fouling as each subsequent bullet passes. It's suppose to be a bear to remove, necessitating all sorts of deadly chemical methods to accomplish the task. I don't know about all this but only read it. "Hatcher's Notebook" and some of Townshend Whelen's works are two reference sources that come to mind. I never shot enough at a time to encounter a problem.
Another potential problem I witnessed first hand is quality control issues resulting from hurried wartime production. In this instance, a case rupture.
I have a friend who was given his deceased uncle's M1 years ago. The uncle was career Army and had served in World War II. The M1 had been left at my friend's grandmother's house. While I was there for a visit she suggested that he might just take the rifle home with him.
It was a really nice, all correct and matching M1 from the World War II era. There was a quantity of Frankfort Arsenal ammunition with a wartime dated head stamp with it in the closet with the rifle. Later that day we went out in my friend's yard, set up some impromptu targets, with a hill backing, and broke out the M1 and ammo. We took turns firing it and exclaiming over its wonders. While my friend was firing it the side of the rifle exploded in a spectacular fashion. I was standing only about 18 inches to his immediate left and was showered in gas, particles, and wood splinters without being hurt at all. My friend wasn't hurt either but the rifle was a sorry mess.
Seems a case head cracked, likely due to not being properly annealed during manufacture. The culprit case was examined and was found to be missing the primer and with a crack leading from the flash hole across the head stamp and up the side of the case.
In the end only the stock itself was ruined and it had been a nicely marked stock. We were certainly through with that rifle for the day. I suggested that he at least clean the rifle after use with corrosively primed ammunition but he said he'd do it later that evening.
Months later he came for a visit and brought me the rifle to sell for him at the Dallas Market Hall gun show. I'd found him a generic replacement stock and we put the rifle in it. A look down the bore was a shock. He had not cleaned it after that afternoon and the bore was a pitted ruin. Only one single outing with corrosive ammunition had devastated the rifle.
I only got a few hundred for it, a rifle that even back then would have brought a premium because of its originality.
Most reasonably modern arms could handle most glitches that could arise from using old ammunition such as neck splits, hang fires, pierced primers, or other impaired performance. With a proper cleaning, the guns would be fine after firing with black powder, corrosive priming, or cupro-nickel jackets. Our fore fathers had to deal with such issues and got by fine.
A case rupture would be another thing entirely and damage will likely result.
I did attempt to fire a couple of Frankfort Arsenal '26 .45 ACP cartridges only a few months back while I was chronographing some other handgun loads. I took a photo of the 3 rounds I'd scrounged up from a local gun show. After snapping the photo I tried firing them over the chronograph screens. Neither of the two fired that I tried and the third one was saved for the cartridge collection.