Yesterday, I fired 4 expired "Orion" 12 guage flares. All four fired
but only one worked right. Results

expiration year results
1990 Worked correctly
1995 Fired but failed to ignite flare
1998 Fired and flare ignited but only went to 50'
altitude
1998 Fired but flare failed to ignite

2 Current flares: Fired and worked correctly.

Two years ago, on a very rainy New Yrs, I fired a bunch of expired
flares and all fired. However, the rain was so intense that I was
unable to determine if the flares ignited.

This test tells me that although old flares ussually fire, they do not
work well.