A very good article was run on this subject in the American Rifleman some years back when they were still shooter rather than political orientated. The conclusion was any dangerous pressures were extremely unlikely to occur. One thing to be cautious of is primer deterioration. Failure of the primer to properly ignite the powder can leave an obstruction in the bore. Smokeless Powder can under some conditions deteriorate as well but is far more likely to give reduced performance than to give a dangerous load. Black Powder is virtually unchanged with age unless it becomes "Wet". Remember the old muzzle loader adage "Keep Your Powder Dry".
If you do choose to shoot old ammunition if any sign of anything abnormal occurs "DO" check the bore before firing another round, just may save your barrel or your hand from an obstructional burst. The "Bad" round is not apt to blow anything up, the next one might if something is left in the bore.
