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Pyrodex has a pretty good reputation for stability, if stored properly in the original container.
BP just lasts. I've had no trouble with year-old quickloads or powder kept in a good flask. I would never trust 777 that had been open for even a year except for practice.
In my experiences Pyrodex lasts poorly if exposed to atmospheric conditions. I have had trouble with it absorbing moisture, FTF.
This is the first I had heard of long-term stability issues with 777. Not encouraging, as I have a few lbs squirreled away for a rainy day.