One thing shooters should watch for with old shells is "clumping" of shot and expanded base wads. If the shot gets damp or wet enough, the shot can corrode and basically rust weld from oxidation and form a single projectile. Wet base wads can expand and raise pressures.
Thus said, I assume ya'll know this already but here is my ten cents worth, adjusting for inflation.
I have cut into old paper PETERS 16 which looked to be in good condition and discovered the lead shot oxidized and clumpy; though not a solid clump. The clumps were loose and I believe they would likely break up on firing. While I believe it possible to end up with a solid mass, any concerns could be addressed by cutting a few samples from the pile of a particular shell and opening them to see and allay concerns.
Having said that on lead shotshells I would be more concerned with steel shotshells as many of them are already at the high end of pressure and a rusty mass clumped up could be devastating. Last year I went to use some 16 steel from the 80s and it was apparent through the plastic they were rusty and a little swelled. I threw them away as It was not worth the bother.
