Years back I bought up a bunch of Pyrodex pellets, off a clearance rack at Wally World, thought I was set for life, at a bargain price. Flash forward 3 or 4 years and I was shooting my .50 cal. to check the zero before deer season. It shot some crazy groups, all over the paper and had a couple of shoots that barely shot at all. I was very confused about it because this gun and pellet combo had been shooting outstanding groups. Finally contacted a very knowledgeable muzzleloader I knew and asked him to take a look at it. Before he would look at it, he asked me what I was shooting, I said Pyrodex pellets, he asked how old the pellets were and I said at least 4 or 5 years. He allowed he didn't have to look at it, the pellets had deteriorated and told me to get some black powered, loose Pyrodex or 777 or some Blackhorn 209 (this was about the time it had just come out) and it would fix my problem. He said he saw it all the time and not just with folks like me that bought up a bunch and stored it, but with folks that thought they bought fresh, but what they bought had been sitting on a shelf or in storage for years and they just figured it was fresh.
I haven't shot pellets since.