That is what I thought also Jon, as primer and blasting cap comp detonates and does not merely burn rapidly like high explosives.
I guess very old or improperly stored primers could indeed become 'weak', and cause bloopers, but this must be very uncommon compared to all the other causative factors.
For the record Der Ami all my loads were 2 3/4" 16ga with an ounce of 8's in Federal paper and plastic cases. Used a variety of primers and mostly Rem. and Ballistic Products shotcups. I looked in my old notebook:
1986-1995: 2 1/2 dr DuPont Bulk Smokeless (hated to lose my old yellow friend that my dad and I also used for years in muzzleloading shotguns and rifles as well).
1995-2004: 22.2 grains 4756
2004-present: 20 grains 7625
As I look back, my bloopers seemed to be caused by pellet loss from loose or damaged crimps (I used to drop candle wax into the centers if I could see pellets), moisture, and powder loss from loose primers. Dishing of the base sometimes occurred and I had to reform the bases and primer pockets on a few hundred cases.
