Old Ammo
In the mid-'60's, as an impoverished college student, I was given a box of WWII-era steel (zinc?) based, paper-cased, roll-crimed 12-ga shotgun ammo, in its original box, the label of which assured the user that the ammo complied with the wartime regs on conserving vital metals. Therefore at the time it was <> 20 years old. (It was a nationally recognized brand; don't remember which.) Of course, as an impoverished college student, I shot up the ammo at various birds, squirrels, and rabbits, then discarded the box. I recall 2 FTF's out of the 25 rounds, one of which cost me a nice bunny. Would that I had stored that box of ammo away—think what a collector would pay for it today! But then, that's true of most of the stuff I used up, broke, wore out, or threw away in my early years, and I'm engaged in the business of living, not running a freaking museum. So I waste very little effort regretting this.
The point is, that the ammo hadn't been stored in any special way except not damp, it all chambered, and 92% worked. And paper shotshells are much less protective of the load inside than any other pre-loaded ammo IMHO.
I would not hesitate to fire any modern ammo, in a modern firearm chambered for same, if the firearm were in good condition and the cartridge(s) showed no sign of bulging, corrosion, splits or other obvious malfunctions. (An old gun, however, a stranger to me, I would have checked out by a competent gunsmith first of course. And everyone's definition of "modern" is different.) I WOULD suspect old rifle ammo of having corrosive primers, and clean my weapon accordingly after such firing. And as noted above, I would pay attention for squids—oops—squibs.
BTW, most ammo is shipped by semi-truck, and the powder gets "tumbled" all the way from manufacturer to shore shelf. Same with reloading powder you buy or order by the can.
I've begun tumbling all my reloaded rifle and pistol ammo, to clean off any lube left on the cases. A side benefit is that it really looks nice. Have observed no difference in ballistic performance. The rounds chamber and handle really slick, and I suppose that my bottle-neck rifle cases grip the chamber walls better (but have no way of measuring this.)