Surplus Ammo
We shoot surplus ammo (some from the 40s) all the time in our Mausers, Mosin Nagants, Springfields and Lee Enfields. I've often shot US Government issued ammo in DCM/CMP rifle matches that dated back to the early 50s and when I was growing up, it was not unusual to see pre-WWII ammo in the hardware store and at the range.
Most military ammo was stored properly while in the custody of the military and many of the components used in manufacturing the ammo were selected for their stability for long term storage. That is why military ammo of the 50s and even into the 60s still have corrosive primers. The corrosive primers were more stable over time than the newly developed non-corrosive types available back then.
The brass cases, if not annealed properly at the neck will often age harden. You can often see stress induced split necks in old ammo, and like JoeW notes, case head separation is also possible from too hard brass. The brass case needs to be pliable enough to obturate (expand) to fit the chamber walls to prevent the gas (often over 50,000 lbs per square inch) from blowing back past the case into the action.
While I've never seen any injuries from this, wearing safety glasses while shooting is a good idea in the event a case head separates and you get gas blow back through the rifle's action and into your face. Some rifles, like the Model 98 Mauser, handle the high pressure gas better than others. In fact, Peter and Paul Mauser designed their opus magnum that way because components used in the new fangled-smokeles-high-pressure ammo of the 1890s was prone to failures such as JoeW notes.
The biggest issue with old surplus ammo is the corrosive priming salts left in the barrel after firing. It is always a good idea to clean the bore as soon as possible with hot water and detergent or Windex with viniger to wash the salts out. Clean a couple of times over the course of a week and use a good rust preventative greese like "Rig" when done. Use the same cleaning regime as for black powder weapons and your rifle will last a long time with a good bore. Neglect this, and your bore may rust to the point of looking like a hundred miles of bad road. Soldiers were required to clean their weapons thoroughly after firing; but many civilians buying the surplus weapons, many used to non-corrosive civilian ammo, failed to do so and that's why you often see very nice surplus rifles at gun shows with horrible bores.
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